6种常用Bean拷贝工具 - BeanUtils、BeanCopier、Dozer、Orika性能比较 - 《Java 笔记》

背景
性能分析
Spring的BeanUtils
Apache的BeanUtils
BeanCopier
Dozer
Orika
总结

Java BeanUtils BeanCopier Dozer Orika

背景

在分层的代码架构中，层与层之间的对象避免不了要做很多转换、赋值等操作，这些操作重复且繁琐，于是乎催生出很多工具来优雅，高效地完成这个操作，有BeanUtils、BeanCopier、Dozer、Orika等等，对上面几个工具的使用、性能对比及原理分析。

性能分析

其实这几个工具要做的事情很简单，而且在使用上也是类似的，先看看性能分析的对比结果，有一个大概的认识。使用JMH来做性能分析的，代码如下：
要复制的对象比较简单，包含了一些基本类型；有一次warmup，因为一些工具是需要“预编译”和做缓存的，这样做对比才会比较客观；分别复制1000、10000、100000个对象，这是比较常用数量级了吧。

@BenchmarkMode(Mode.AverageTime)  
@OutputTimeUnit(TimeUnit.MICROSECONDS)  
@Fork(1)  
@Warmup(iterations = 1)  
@State(Scope.Benchmark)  
public class BeanMapperBenchmark {  
    @Param({"1000", "10000", "100000"})  
    private int times;  
    private int time;  
    private static MapperFactory mapperFactory;  
    private static Mapper mapper;  
    static {  
        mapperFactory = new DefaultMapperFactory.Builder().build();  
        mapperFactory.classMap(SourceVO.class, TargetVO.class)  
            .byDefault()  
            .register();  
        mapper = DozerBeanMapperBuilder.create()  
            .withMappingBuilder(new BeanMappingBuilder() {  
                @Override  
                protected void configure() {  
                    mapping(SourceVO.class, TargetVO.class)  
                        .fields("fullName", "name")  
                        .exclude("in");  
                }  
            }).build();  
    }  
    public static void main(String[] args) throws Exception {  
        Options options = new OptionsBuilder()  
            .include(BeanMapperBenchmark.class.getName()).measurementIterations(3)  
            .build();  
        new Runner(options).run();  
    }  
    @Setup  
    public void prepare() {  
        this.time = times;  
    }  
    @Benchmark  
    public void springBeanUtilTest(){  
        SourceVO sourceVO = getSourceVO();  
        for(int i = 0; i < time; i++){  
            TargetVO targetVO = new TargetVO();  
            BeanUtils.copyProperties(sourceVO, targetVO);  
        }  
    }  
    @Benchmark  
    public void apacheBeanUtilTest() throws Exception{  
        SourceVO sourceVO = getSourceVO();  
        for(int i = 0; i < time; i++){  
            TargetVO targetVO = new TargetVO();  
            org.apache.commons.beanutils.BeanUtils.copyProperties(targetVO, sourceVO);  
        }  
    }  
    @Benchmark  
    public void beanCopierTest(){  
        SourceVO sourceVO = getSourceVO();  
        for(int i = 0; i < time; i++){  
            TargetVO targetVO = new TargetVO();  
            BeanCopier bc = BeanCopier.create(SourceVO.class, TargetVO.class, false);  
            bc.copy(sourceVO, targetVO, null);  
        }  
    }  
    @Benchmark  
    public void dozerTest(){  
        SourceVO sourceVO = getSourceVO();  
        for(int i = 0; i < time; i++){  
            TargetVO map = mapper.map(sourceVO, TargetVO.class);  
        }  
    }  
    @Benchmark  
    public void orikaTest(){  
        SourceVO sourceVO = getSourceVO();  
        for(int i = 0; i < time; i++){  
            MapperFacade mapper = mapperFactory.getMapperFacade();  
            TargetVO map = mapper.map(sourceVO, TargetVO.class);  
        }  
    }  
    private SourceVO getSourceVO(){  
        SourceVO sourceVO = new SourceVO();  
        sourceVO.setP1(1);  
        sourceVO.setP2(2L);  
        sourceVO.setP3(new Integer(3).byteValue());  
        sourceVO.setDate1(new Date());  
        sourceVO.setPattr1("1");  
        sourceVO.setIn(new SourceVO.Inner(1));  
        sourceVO.setFullName("alben");  
        return sourceVO;  
    }  
}

运行后的结果如下：

Score表示的是平均运行时间，单位是微秒。从执行效率来看，可以看出 beanCopier > orika > Spring BeanUtil > dozer > Apache BeanUtil。这样的结果跟它们各自的实现原理有很大的关系，
下面将详细每个工具的使用及实现原理。

Spring的`BeanUtils`

使用

这个工具可能是大家日常使用最多的，因为是Spring自带的，使用也简单：

BeanUtils.copyProperties(sourceVO, targetVO);

原理

Spring BeanUtils的实现原理也比较简答，就是通过Java的Introspector获取到两个类的PropertyDescriptor，对比两个属性具有相同的名字和类型，如果是，则进行赋值（通过ReadMethod获取值，通过WriteMethod赋值），否则忽略。
为了提高性能Spring对BeanInfo和PropertyDescriptor进行了缓存。
（源码基于：org.springframework:spring-beans:4.3.9.RELEASE）

/**  
  * Copy the property values of the given source bean into the given target bean.  
  * <p>Note: The source and target classes do not have to match or even be derived  
  * from each other, as long as the properties match. Any bean properties that the  
  * source bean exposes but the target bean does not will silently be ignored.  
  * @param source the source bean  
  * @param target the target bean  
  * @param editable the class (or interface) to restrict property setting to  
  * @param ignoreProperties array of property names to ignore  
  * @throws BeansException if the copying failed  
  * @see BeanWrapper  
  */  
private static void copyProperties(Object source, Object target, Class<?> editable, String... ignoreProperties)  
    throws BeansException {  
    Assert.notNull(source, "Source must not be null");  
    Assert.notNull(target, "Target must not be null");  
    Class<?> actualEditable = target.getClass();  
    if (editable != null) {  
        if (!editable.isInstance(target)) {  
            throw new IllegalArgumentException("Target class [" + target.getClass().getName() +  
                                               "] not assignable to Editable class [" + editable.getName() + "]");  
        }  
        actualEditable = editable;  
    }  
    //获取target类的属性（有缓存）  
    PropertyDescriptor[] targetPds = getPropertyDescriptors(actualEditable);  
    List<String> ignoreList = (ignoreProperties != null ? Arrays.asList(ignoreProperties) : null);  
    for (PropertyDescriptor targetPd : targetPds) {  
        Method writeMethod = targetPd.getWriteMethod();  
        if (writeMethod != null && (ignoreList == null || !ignoreList.contains(targetPd.getName()))) {  
            //获取source类的属性（有缓存）  
            PropertyDescriptor sourcePd = getPropertyDescriptor(source.getClass(), targetPd.getName());  
            if (sourcePd != null) {  
                Method readMethod = sourcePd.getReadMethod();  
                if (readMethod != null &&  
                    //判断target的setter方法入参和source的getter方法返回类型是否一致  
                    ClassUtils.isAssignable(writeMethod.getParameterTypes()[0], readMethod.getReturnType())) {  
                    try {  
                        if (!Modifier.isPublic(readMethod.getDeclaringClass().getModifiers())) {  
                            readMethod.setAccessible(true);  
                        }  
                        //获取源值  
                        Object value = readMethod.invoke(source);  
                        if (!Modifier.isPublic(writeMethod.getDeclaringClass().getModifiers())) {  
                            writeMethod.setAccessible(true);  
                        }  
                        //赋值到target  
                        writeMethod.invoke(target, value);  
                    }  
                    catch (Throwable ex) {  
                        throw new FatalBeanException(  
                            "Could not copy property '" + targetPd.getName() + "' from source to target", ex);  
                    }  
                }  
            }  
        }  
    }  
}

小结

Spring BeanUtils的实现就是这么简洁，这也是它性能比较高的原因。
不过，过于简洁就失去了灵活性和可扩展性了，Spring BeanUtils的使用限制也比较明显，要求类属性的名字和类型一致，这点在使用时要注意。

Apache的`BeanUtils`

使用

Apache的BeanUtils和Spring的BeanUtils的使用是一样的：

BeanUtils.copyProperties(targetVO, sourceVO);

要注意，source和target的入参位置不同。

原理

Apache的BeanUtils的实现原理跟Spring的BeanUtils一样，也是主要通过Java的Introspector机制获取到类的属性来进行赋值操作，对BeanInfo和PropertyDescriptor同样有缓存，但是Apache BeanUtils加了一些不那么使用的特性（包括支持Map类型、支持自定义的DynaBean类型、支持属性名的表达式等等）在里面，使得性能相对Spring的BeanUtils来说有所下降。
（源码基于：commons-beanutils:commons-beanutils:1.9.3）

public void copyProperties(final Object dest, final Object orig)  
    throws IllegalAccessException, InvocationTargetException {  
    if (dest == null) {  
        throw new IllegalArgumentException  
            ("No destination bean specified");  
    }  
    if (orig == null) {  
        throw new IllegalArgumentException("No origin bean specified");  
    }  
    if (log.isDebugEnabled()) {  
        log.debug("BeanUtils.copyProperties(" + dest + ", " +  
                  orig + ")");  
    }  
    // Apache Common自定义的DynaBean  
    if (orig instanceof DynaBean) {  
        final DynaProperty[] origDescriptors =  
            ((DynaBean) orig).getDynaClass().getDynaProperties();  
        for (DynaProperty origDescriptor : origDescriptors) {  
            final String name = origDescriptor.getName();  
            // Need to check isReadable() for WrapDynaBean  
            // (see Jira issue# BEANUTILS-61)  
            if (getPropertyUtils().isReadable(orig, name) &&  
                getPropertyUtils().isWriteable(dest, name)) {  
                final Object value = ((DynaBean) orig).get(name);  
                copyProperty(dest, name, value);  
            }  
        }  
        // Map类型  
    } else if (orig instanceof Map) {  
        @SuppressWarnings("unchecked")  
        final  
            // Map properties are always of type <String, Object>  
            Map<String, Object> propMap = (Map<String, Object>) orig;  
        for (final Map.Entry<String, Object> entry : propMap.entrySet()) {  
            final String name = entry.getKey();  
            if (getPropertyUtils().isWriteable(dest, name)) {  
                copyProperty(dest, name, entry.getValue());  
            }  
        }  
        // 标准的JavaBean  
    } else {  
        final PropertyDescriptor[] origDescriptors =  
            //获取PropertyDescriptor  
            getPropertyUtils().getPropertyDescriptors(orig);  
        for (PropertyDescriptor origDescriptor : origDescriptors) {  
            final String name = origDescriptor.getName();  
            if ("class".equals(name)) {  
                continue; // No point in trying to set an object's class  
            }  
            //是否可读和可写  
            if (getPropertyUtils().isReadable(orig, name) &&  
                getPropertyUtils().isWriteable(dest, name)) {  
                try {  
                    //获取源值  
                    final Object value =  
                        getPropertyUtils().getSimpleProperty(orig, name);  
                    //赋值操作  
                    copyProperty(dest, name, value);  
                } catch (final NoSuchMethodException e) {  
                    // Should not happen  
                }  
            }  
        }  
    }  
}

小结

Apache BeanUtils的实现跟Spring BeanUtils总体上类似，但是性能却低很多，这个可以从上面性能比较看出来。阿里的Java规范是不建议使用的。

`BeanCopier`

使用

BeanCopier在cglib包里，它的使用也比较简单：

@Test  
public void beanCopierSimpleTest() {  
    SourceVO sourceVO = getSourceVO();  
    log.info("source={}", GsonUtil.toJson(sourceVO));  
    TargetVO targetVO = new TargetVO();  
    BeanCopier bc = BeanCopier.create(SourceVO.class, TargetVO.class, false);  
    bc.copy(sourceVO, targetVO, null);  
    log.info("target={}", GsonUtil.toJson(targetVO));  
}

只需要预先定义好要转换的source类和target类就好了，可以选择是否使用Converter，这个下面会说到。
在上面的性能测试中，BeanCopier是所有中表现最好的，那么分析一下它的实现原理。

原理

BeanCopier的实现原理跟BeanUtils截然不同，它不是利用反射对属性进行赋值，而是直接使用cglib来生成带有的get/set方法的class类，然后执行。由于是直接生成字节码执行，所以BeanCopier的性能接近手写
get/set。
BeanCopier.create方法

public static BeanCopier create(Class source, Class target, boolean useConverter) {  
    Generator gen = new Generator();  
    gen.setSource(source);  
    gen.setTarget(target);  
    gen.setUseConverter(useConverter);  
    return gen.create();  
}  
public BeanCopier create() {  
 Object key = KEY_FACTORY.newInstance(source.getName(), target.getName(), useConverter);  
 return (BeanCopier)super.create(key);  
}

这里的意思是用KEY_FACTORY创建一个BeanCopier出来，然后调用create方法来生成字节码。
KEY_FACTORY其实就是用cglib通过BeanCopierKey接口生成出来的一个类

private static final BeanCopierKey KEY_FACTORY =  (BeanCopierKey)KeyFactory.create(BeanCopierKey.class);  
interface BeanCopierKey {  
 public Object newInstance(String source, String target, boolean useConverter);  
}

通过设置

System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "path");

可以让cglib输出生成类的class文件，可以反编译看看里面的代码
下面是KEY_FACTORY的类

public class BeanCopier$BeanCopierKey$$KeyFactoryByCGLIB$$f32401fd extends KeyFactory implements BeanCopierKey {  
    private final String FIELD_0;  
    private final String FIELD_1;  
    private final boolean FIELD_2;  
    public BeanCopier$BeanCopierKey$$KeyFactoryByCGLIB$$f32401fd() {  
    }  
    public Object newInstance(String var1, String var2, boolean var3) {  
        return new BeanCopier$BeanCopierKey$$KeyFactoryByCGLIB$$f32401fd(var1, var2, var3);  
    }  
    public BeanCopier$BeanCopierKey$$KeyFactoryByCGLIB$$f32401fd(String var1, String var2, boolean var3) {  
        this.FIELD_0 = var1;  
        this.FIELD_1 = var2;  
        this.FIELD_2 = var3;  
    }  
    //省去hashCode等方法。。。  
}

继续跟踪Generator.create方法，由于Generator是继承AbstractClassGenerator，这个AbstractClassGenerator是cglib用来生成字节码的一个模板类，Generator的super.create其实调用AbstractClassGenerator的create方法，最终会调用到Generator的模板方法generateClass方法，不去细究AbstractClassGenerator的细节，重点看generateClass。
这个是一个生成java类的方法，理解起来就好像平时写代码一样。

public void generateClass(ClassVisitor v) {  
    Type sourceType = Type.getType(source);  
    Type targetType = Type.getType(target);  
    ClassEmitter ce = new ClassEmitter(v);  
    //开始“写”类，这里有修饰符、类名、父类等信息  
    ce.begin_class(Constants.V1_2,  
                   Constants.ACC_PUBLIC,  
                   getClassName(),  
                   BEAN_COPIER,  
                   null,  
                   Constants.SOURCE_FILE);  
    //没有构造方法  
    EmitUtils.null_constructor(ce);  
    //开始“写”一个方法，方法名是copy  
    CodeEmitter e = ce.begin_method(Constants.ACC_PUBLIC, COPY, null);  
    //通过Introspector获取source类和target类的PropertyDescriptor  
    PropertyDescriptor[] getters = ReflectUtils.getBeanGetters(source);  
    PropertyDescriptor[] setters = ReflectUtils.getBeanSetters(target);  
    Map names = new HashMap();  
    for (int i = 0; i < getters.length; i++) {  
        names.put(getters[i].getName(), getters[i]);  
    }  
    Local targetLocal = e.make_local();  
    Local sourceLocal = e.make_local();  
    if (useConverter) {  
        e.load_arg(1);  
        e.checkcast(targetType);  
        e.store_local(targetLocal);  
        e.load_arg(0);                  
        e.checkcast(sourceType);  
        e.store_local(sourceLocal);  
    } else {  
        e.load_arg(1);  
        e.checkcast(targetType);  
        e.load_arg(0);  
        e.checkcast(sourceType);  
    }  
    //通过属性名来生成转换的代码  
    //以setter作为遍历  
    for (int i = 0; i < setters.length; i++) {  
        PropertyDescriptor setter = setters[i];  
        //根据setter的name获取getter  
        PropertyDescriptor getter = (PropertyDescriptor)names.get(setter.getName());  
        if (getter != null) {  
            //获取读写方法  
            MethodInfo read = ReflectUtils.getMethodInfo(getter.getReadMethod());  
            MethodInfo write = ReflectUtils.getMethodInfo(setter.getWriteMethod());  
            //如果用了useConverter，则进行下面的拼装代码方式  
            if (useConverter) {  
                Type setterType = write.getSignature().getArgumentTypes()[0];  
                e.load_local(targetLocal);  
                e.load_arg(2);  
                e.load_local(sourceLocal);  
                e.invoke(read);  
                e.box(read.getSignature().getReturnType());  
                EmitUtils.load_class(e, setterType);  
                e.push(write.getSignature().getName());  
                e.invoke_interface(CONVERTER, CONVERT);  
                e.unbox_or_zero(setterType);  
                e.invoke(write);  
                //compatible用来判断getter和setter是否类型一致  
            } else if (compatible(getter, setter)) {  
                e.dup2();  
                e.invoke(read);  
                e.invoke(write);  
            }  
        }  
    }  
    e.return_value();  
    e.end_method();  
    ce.end_class();  
}  
private static boolean compatible(PropertyDescriptor getter, PropertyDescriptor setter) {  
    // TODO: allow automatic widening conversions?  
    return setter.getPropertyType().isAssignableFrom(getter.getPropertyType());  
}

即使没有使用过cglib也能读懂生成代码的流程吧，看看没有使用useConverter的情况下生成的代码：

public class Object$$BeanCopierByCGLIB$$d1d970c8 extends BeanCopier {  
    public Object$$BeanCopierByCGLIB$$d1d970c8() {  
    }  
    public void copy(Object var1, Object var2, Converter var3) {  
        TargetVO var10000 = (TargetVO)var2;  
        SourceVO var10001 = (SourceVO)var1;  
        var10000.setDate1(((SourceVO)var1).getDate1());  
        var10000.setIn(var10001.getIn());  
        var10000.setListData(var10001.getListData());  
        var10000.setMapData(var10001.getMapData());  
        var10000.setP1(var10001.getP1());  
        var10000.setP2(var10001.getP2());  
        var10000.setP3(var10001.getP3());  
        var10000.setPattr1(var10001.getPattr1());  
    }  
}

在对比上面生成代码的代码是不是阔然开朗了。
再看看使用useConverter的情况：

public class Object$$BeanCopierByCGLIB$$d1d970c7 extends BeanCopier {  
    private static final Class CGLIB$load_class$java$2Eutil$2EDate;  
    private static final Class CGLIB$load_class$beanmapper_compare$2Evo$2ESourceVO$24Inner;  
    private static final Class CGLIB$load_class$java$2Eutil$2EList;  
    private static final Class CGLIB$load_class$java$2Eutil$2EMap;  
    private static final Class CGLIB$load_class$java$2Elang$2EInteger;  
    private static final Class CGLIB$load_class$java$2Elang$2ELong;  
    private static final Class CGLIB$load_class$java$2Elang$2EByte;  
    private static final Class CGLIB$load_class$java$2Elang$2EString;  
    public Object$$BeanCopierByCGLIB$$d1d970c7() {  
    }  
    public void copy(Object var1, Object var2, Converter var3) {  
        TargetVO var4 = (TargetVO)var2;  
        SourceVO var5 = (SourceVO)var1;  
        var4.setDate1((Date)var3.convert(var5.getDate1(), CGLIB$load_class$java$2Eutil$2EDate, "setDate1"));  
        var4.setIn((Inner)var3.convert(var5.getIn(), CGLIB$load_class$beanmapper_compare$2Evo$2ESourceVO$24Inner, "setIn"));  
        var4.setListData((List)var3.convert(var5.getListData(), CGLIB$load_class$java$2Eutil$2EList, "setListData"));  
        var4.setMapData((Map)var3.convert(var5.getMapData(), CGLIB$load_class$java$2Eutil$2EMap, "setMapData"));  
        var4.setP1((Integer)var3.convert(var5.getP1(), CGLIB$load_class$java$2Elang$2EInteger, "setP1"));  
        var4.setP2((Long)var3.convert(var5.getP2(), CGLIB$load_class$java$2Elang$2ELong, "setP2"));  
        var4.setP3((Byte)var3.convert(var5.getP3(), CGLIB$load_class$java$2Elang$2EByte, "setP3"));  
        var4.setPattr1((String)var3.convert(var5.getPattr1(), CGLIB$load_class$java$2Elang$2EString, "setPattr1"));  
        var4.setSeq((Long)var3.convert(var5.getSeq(), CGLIB$load_class$java$2Elang$2ELong, "setSeq"));  
    }  
    static void CGLIB$STATICHOOK1() {  
        CGLIB$load_class$java$2Eutil$2EDate = Class.forName("java.util.Date");  
        CGLIB$load_class$beanmapper_compare$2Evo$2ESourceVO$24Inner = Class.forName("beanmapper_compare.vo.SourceVO$Inner");  
        CGLIB$load_class$java$2Eutil$2EList = Class.forName("java.util.List");  
        CGLIB$load_class$java$2Eutil$2EMap = Class.forName("java.util.Map");  
        CGLIB$load_class$java$2Elang$2EInteger = Class.forName("java.lang.Integer");  
        CGLIB$load_class$java$2Elang$2ELong = Class.forName("java.lang.Long");  
        CGLIB$load_class$java$2Elang$2EByte = Class.forName("java.lang.Byte");  
        CGLIB$load_class$java$2Elang$2EString = Class.forName("java.lang.String");  
    }  
    static {  
        CGLIB$STATICHOOK1();  
    }  
}

小结

BeanCopier性能确实很高，但从源码可以看出BeanCopier只会拷贝名称和类型都相同的属性，而且如果一旦使用Converter，BeanCopier只使用Converter定义的规则去拷贝属性，所以在convert方法中要考虑所有的属性。

`Dozer`

使用

上面提到的BeanUtils和BeanCopier都是功能比较简单的，需要属性名称一样，甚至类型也要一样。但是在大多数情况下这个要求就相对苛刻了，要知道有些VO由于各种原因不能修改，有些是外部接口SDK的对象，
有些对象的命名规则不同，例如有驼峰型的，有下划线的等等，各种什么情况都有。所以更加需要的是更加灵活丰富的功能，甚至可以做到定制化的转换。
Dozer就提供了这些功能，有支持同名隐式映射，支持基本类型互相转换，支持显示指定映射关系，支持exclude字段，支持递归匹配映射，支持深度匹配，支持Date to String的date-formate，支持自定义转换Converter，支持一次mapping定义多处使用，支持EventListener事件监听等等。不仅如此，Dozer在使用方式上，除了支持API，还支持XML和注解，满足大家的喜好。
由于其功能很丰富，不可能每个都演示，这里只是给个大概认识，更详细的功能，或者XML和注解的配置，请看官方文档。

private Mapper dozerMapper;  
@Before  
public void setup(){  
    dozerMapper = DozerBeanMapperBuilder.create()  
    .withMappingBuilder(new BeanMappingBuilder() {  
        @Override  
        protected void configure() {  
            mapping(SourceVO.class, TargetVO.class)  
            .fields("fullName", "name")  
            .exclude("in");  
        }  
    })  
    .withCustomConverter(null)  
    .withEventListener(null)  
    .build();  
}  
@Test  
public void dozerTest(){  
    SourceVO sourceVO = getSourceVO();  
    log.info("sourceVO={}", GsonUtil.toJson(sourceVO));  
    TargetVO map = dozerMapper.map(sourceVO, TargetVO.class);  
    log.info("map={}", GsonUtil.toJson(map));  
}

原理

Dozer的实现原理本质上还是用反射/Introspector那套，但是其丰富的功能，以及支持多种实现方式（API、XML、注解）使得代码看上去有点复杂，在翻阅代码时，大可不必理会这些类，只需要知道它们大体的作用就行了，重点关注核心流程和代码的实现。下面重点看看构建mapper的build方法和实现映射的map方法。
build方法很简单，它是一个初始化的动作，就是通过用户的配置来构建出一系列后面要用到的配置对象、上下文对象，或其他封装对象，不必深究这些对象是怎么实现的，从名字上大概能猜出这些对象是干嘛，负责什么就可以了。

DozerBeanMapper(List<String> mappingFiles,  
                BeanContainer beanContainer,  
                DestBeanCreator destBeanCreator,  
                DestBeanBuilderCreator destBeanBuilderCreator,  
                BeanMappingGenerator beanMappingGenerator,  
                PropertyDescriptorFactory propertyDescriptorFactory,  
                List<CustomConverter> customConverters,  
                List<MappingFileData> mappingsFileData,  
                List<EventListener> eventListeners,  
                CustomFieldMapper customFieldMapper,  
                Map<String, CustomConverter> customConvertersWithId,  
                ClassMappings customMappings,  
                Configuration globalConfiguration,  
                CacheManager cacheManager) {  
    this.beanContainer = beanContainer;  
    this.destBeanCreator = destBeanCreator;  
    this.destBeanBuilderCreator = destBeanBuilderCreator;  
    this.beanMappingGenerator = beanMappingGenerator;  
    this.propertyDescriptorFactory = propertyDescriptorFactory;  
    this.customConverters = new ArrayList<>(customConverters);  
    this.eventListeners = new ArrayList<>(eventListeners);  
    this.mappingFiles = new ArrayList<>(mappingFiles);  
    this.customFieldMapper = customFieldMapper;  
    this.customConvertersWithId = new HashMap<>(customConvertersWithId);  
    this.eventManager = new DefaultEventManager(eventListeners);  
    this.customMappings = customMappings;  
    this.globalConfiguration = globalConfiguration;  
    this.cacheManager = cacheManager;  
}

map方法是映射对象的过程，其入口是MappingProcessor的mapGeneral方法

private <T> T mapGeneral(Object srcObj, final Class<T> destClass, final T destObj, final String mapId) {  
    srcObj = MappingUtils.deProxy(srcObj, beanContainer);  
    Class<T> destType;  
    T result;  
    if (destClass == null) {  
        destType = (Class<T>)destObj.getClass();  
        result = destObj;  
    } else {  
        destType = destClass;  
        result = null;  
    }  
    ClassMap classMap = null;  
    try {  
        //构建ClassMap  
        //ClassMap是包括src类和dest类和其他配置的一个封装  
        classMap = getClassMap(srcObj.getClass(), destType, mapId);  
        //注册事件  
        eventManager.on(new DefaultEvent(EventTypes.MAPPING_STARTED, classMap, null, srcObj, result, null));  
        //看看有没有自定义converter  
        Class<?> converterClass = MappingUtils.findCustomConverter(converterByDestTypeCache, classMap.getCustomConverters(), srcObj  
                                                                   .getClass(), destType);  
        if (destObj == null) {  
            // If this is a nested MapperAware conversion this mapping can be already processed  
            // but we can do this optimization only in case of no destObject, instead we must copy to the dest object  
            Object alreadyMappedValue = mappedFields.getMappedValue(srcObj, destType, mapId);  
            if (alreadyMappedValue != null) {  
                return (T)alreadyMappedValue;  
            }  
        }  
        //优先使用自定义converter进行映射  
        if (converterClass != null) {  
            return (T)mapUsingCustomConverter(converterClass, srcObj.getClass(), srcObj, destType, result, null, true);  
        }  
        //也是对配置进行了封装  
        BeanCreationDirective creationDirective =  
        new BeanCreationDirective(srcObj, classMap.getSrcClassToMap(), classMap.getDestClassToMap(), destType,  
                                  classMap.getDestClassBeanFactory(), classMap.getDestClassBeanFactoryId(), classMap.getDestClassCreateMethod(),  
                                  classMap.getDestClass().isSkipConstructor());  
        //继续进行映射  
        result = createByCreationDirectiveAndMap(creationDirective, classMap, srcObj, result, false, null);  
    } catch (Throwable e) {  
        MappingUtils.throwMappingException(e);  
    }  
    eventManager.on(new DefaultEvent(EventTypes.MAPPING_FINISHED, classMap, null, srcObj, result, null));  
    return result;  
}

一般情况下createByCreationDirectiveAndMap方法会一直调用到mapFromFieldMap方法，而在没有自定义converter的情况下会调用mapOrRecurseObject方法。
大多数情况下字段的映射会在这个方法做一般的解析

private Object mapOrRecurseObject(Object srcObj, Object srcFieldValue, Class<?> destFieldType, FieldMap fieldMap, Object destObj) {  
    Class<?> srcFieldClass = srcFieldValue != null ? srcFieldValue.getClass() : fieldMap.getSrcFieldType(srcObj.getClass());  
    Class<?> converterClass = MappingUtils.determineCustomConverter(fieldMap, converterByDestTypeCache, fieldMap.getClassMap()  
                                                                    .getCustomConverters(), srcFieldClass, destFieldType);  
    //自定义converter的处理  
    if (converterClass != null) {  
        return mapUsingCustomConverter(converterClass, srcFieldClass, srcFieldValue, destFieldType, destObj, fieldMap, false);  
    }  
    if (srcFieldValue == null) {  
        return null;  
    }  
    String srcFieldName = fieldMap.getSrcFieldName();  
    String destFieldName = fieldMap.getDestFieldName();  
    if (!(DozerConstants.SELF_KEYWORD.equals(srcFieldName) && DozerConstants.SELF_KEYWORD.equals(destFieldName))) {  
        Object alreadyMappedValue = mappedFields.getMappedValue(srcFieldValue, destFieldType, fieldMap.getMapId());  
        if (alreadyMappedValue != null) {  
            return alreadyMappedValue;  
        }  
    }  
    //如果只是浅拷贝则直接返回（可配置）  
    if (fieldMap.isCopyByReference()) {  
        // just get the src and return it, no transformation.  
        return srcFieldValue;  
    }  
    //对Map类型的处理  
    boolean isSrcFieldClassSupportedMap = MappingUtils.isSupportedMap(srcFieldClass);  
    boolean isDestFieldTypeSupportedMap = MappingUtils.isSupportedMap(destFieldType);  
    if (isSrcFieldClassSupportedMap && isDestFieldTypeSupportedMap) {  
        return mapMap(srcObj, (Map<?, ?>)srcFieldValue, fieldMap, destObj);  
    }  
    if (fieldMap instanceof MapFieldMap && destFieldType.equals(Object.class)) {  
        destFieldType = fieldMap.getDestHintContainer() != null ? fieldMap.getDestHintContainer().getHint() : srcFieldClass;  
    }  
    //对基本类型的映射处理  
    //PrimitiveOrWrapperConverter类支持兼容了基本类型之间的互相转换  
    if (primitiveConverter.accepts(srcFieldClass) || primitiveConverter.accepts(destFieldType)) {  
        // Primitive or Wrapper conversion  
        if (fieldMap.getDestHintContainer() != null) {  
            Class<?> destHintType = fieldMap.getDestHintType(srcFieldValue.getClass());  
            // if the destType is null this means that there was more than one hint.  
            // we must have already set the destType then.  
            if (destHintType != null) {  
                destFieldType = destHintType;  
            }  
        }  
        //#1841448 - if trim-strings=true, then use a trimmed src string value when converting to dest value  
        Object convertSrcFieldValue = srcFieldValue;  
        if (fieldMap.isTrimStrings() && srcFieldValue.getClass().equals(String.class)) {  
            convertSrcFieldValue = ((String)srcFieldValue).trim();  
        }  
        DateFormatContainer dfContainer = new DateFormatContainer(fieldMap.getDateFormat());  
        if (fieldMap instanceof MapFieldMap && !primitiveConverter.accepts(destFieldType)) {  
            return primitiveConverter.convert(convertSrcFieldValue, convertSrcFieldValue.getClass(), dfContainer);  
        } else {  
            return primitiveConverter.convert(convertSrcFieldValue, destFieldType, dfContainer, destFieldName, destObj);  
        }  
    }  
    //对集合类型的映射处理  
    if (MappingUtils.isSupportedCollection(srcFieldClass) && (MappingUtils.isSupportedCollection(destFieldType))) {  
        return mapCollection(srcObj, srcFieldValue, fieldMap, destObj);  
    }  
    //对枚举类型的映射处理  
    if (MappingUtils.isEnumType(srcFieldClass, destFieldType)) {  
        return mapEnum((Enum)srcFieldValue, (Class<Enum>)destFieldType);  
    }  
    if (fieldMap.getDestDeepIndexHintContainer() != null) {  
        destFieldType = fieldMap.getDestDeepIndexHintContainer().getHint();  
    }  
    //其他复杂对象类型的处理  
    return mapCustomObject(fieldMap, destObj, destFieldType, destFieldName, srcFieldValue);  
}

mapCustomObject方法。其实可以发现这个方法最重要的一点就是做递归处理，无论是最后调用createByCreationDirectiveAndMap还是mapToDestObject方法。

private Object mapCustomObject(FieldMap fieldMap, Object destObj, Class<?> destFieldType, String destFieldName, Object srcFieldValue) {  
    srcFieldValue = MappingUtils.deProxy(srcFieldValue, beanContainer);  
    // Custom java bean. Need to make sure that the destination object is not  
    // already instantiated.  
    Object result = null;  
    // in case of iterate feature new objects are created in any case  
    if (!DozerConstants.ITERATE.equals(fieldMap.getDestFieldType())) {  
        result = getExistingValue(fieldMap, destObj, destFieldType);  
    }  
    // if the field is not null than we don't want a new instance  
    if (result == null) {  
        // first check to see if this plain old field map has hints to the actual  
        // type.  
        if (fieldMap.getDestHintContainer() != null) {  
            Class<?> destHintType = fieldMap.getDestHintType(srcFieldValue.getClass());  
            // if the destType is null this means that there was more than one hint.  
            // we must have already set the destType then.  
            if (destHintType != null) {  
                destFieldType = destHintType;  
            }  
        }  
        // Check to see if explicit map-id has been specified for the field  
        // mapping  
        String mapId = fieldMap.getMapId();  
        Class<?> targetClass;  
        if (fieldMap.getDestHintContainer() != null && fieldMap.getDestHintContainer().getHint() != null) {  
            targetClass = fieldMap.getDestHintContainer().getHint();  
        } else {  
            targetClass = destFieldType;  
        }  
        ClassMap classMap = getClassMap(srcFieldValue.getClass(), targetClass, mapId);  
        BeanCreationDirective creationDirective = new BeanCreationDirective(srcFieldValue, classMap.getSrcClassToMap(), classMap.getDestClassToMap(),  
                                                                            destFieldType, classMap.getDestClassBeanFactory(), classMap.getDestClassBeanFactoryId(),  
                                                                            fieldMap.getDestFieldCreateMethod() != null ? fieldMap.getDestFieldCreateMethod() :  
                                                                                    classMap.getDestClassCreateMethod(),  
                                                                            classMap.getDestClass().isSkipConstructor(), destObj, destFieldName);  
        result = createByCreationDirectiveAndMap(creationDirective, classMap, srcFieldValue, null, false, fieldMap.getMapId());  
    } else {  
        mapToDestObject(null, srcFieldValue, result, false, fieldMap.getMapId());  
    }  
    return result;  
}

小结

Dozer功能强大，但底层还是用反射那套，所以在性能测试中它的表现一般，仅次于Apache的BeanUtils。如果不追求性能的话，可以使用。

`Orika`

Orika可以说是几乎集成了上述几个工具的优点，不仅具有丰富的功能，底层使用Javassist生成字节码，运行效率很高的。

使用

Orika基本支持了Dozer支持的功能，这里也是简单介绍一下Orika的使用，具体更详细的API可以参考User Guide。

private MapperFactory mapperFactory;  
@Before  
public void setup() {  
    mapperFactory = new DefaultMapperFactory.Builder().build();  
    ConverterFactory converterFactory = mapperFactory.getConverterFactory();  
    converterFactory.registerConverter(new TypeConverter());  
    mapperFactory.classMap(SourceVO.class, TargetVO.class)  
            .field("fullName", "name")  
            .field("type", "enumType")  
            .exclude("in")  
            .byDefault()  
            .register();  
}  
@Test  
public void main() {  
    MapperFacade mapper = mapperFactory.getMapperFacade();  
    SourceVO sourceVO = getSourceVO();  
    log.info("sourceVO={}", GsonUtil.toJson(sourceVO));  
    TargetVO map = mapper.map(sourceVO, TargetVO.class);  
    log.info("map={}", GsonUtil.toJson(map));  
}

原理

在讲解实现原理时，先看看Orika在背后干了什么事情。
通过增加以下配置，可以看到Orika在做映射过程中生成mapper的源码和字节码。

System.setProperty("ma.glasnost.orika.writeSourceFiles", "true");  
System.setProperty("ma.glasnost.orika.writeClassFiles", "true");  
System.setProperty("ma.glasnost.orika.writeSourceFilesToPath", "path");  
System.setProperty("ma.glasnost.orika.writeClassFilesToPath", "path");

用上面的例子，看看Orika生成的java代码：

package ma.glasnost.orika.generated;  
public class Orika_TargetVO_SourceVO_Mapper947163525829122$0 extends ma.glasnost.orika.impl.GeneratedMapperBase {  
    public void mapAtoB(java.lang.Object a, java.lang.Object b, ma.glasnost.orika.MappingContext mappingContext) {  
        super.mapAtoB(a, b, mappingContext);  
        // sourceType: SourceVO  
        beanmapper_compare.vo.SourceVO source = ((beanmapper_compare.vo.SourceVO)a);   
        // destinationType: TargetVO  
        beanmapper_compare.vo.TargetVO destination = ((beanmapper_compare.vo.TargetVO)b);   
        destination.setName(((java.lang.String)source.getFullName()));   
        if ( !(((java.lang.Integer)source.getType()) == null)){   
            destination.setEnumType(((beanmapper_compare.vo.TargetVO.EnumType)((ma.glasnost.orika.Converter)usedConverters[0]).convert(((java.lang.Integer)source.getType()), ((ma.glasnost.orika.metadata.Type)usedTypes[0]), mappingContext)));   
        } else {   
            destination.setEnumType(null);  
        }  
        if ( !(((java.util.Date)source.getDate1()) == null)){   
            destination.setDate1(((java.util.Date)((ma.glasnost.orika.Converter)usedConverters[1]).convert(((java.util.Date)source.getDate1()), ((ma.glasnost.orika.metadata.Type)usedTypes[1]), mappingContext)));   
        } else {   
            destination.setDate1(null);  
        }if ( !(((java.util.List)source.getListData()) == null)) {  
            java.util.List new_listData = ((java.util.List)new java.util.ArrayList());   
            new_listData.addAll(mapperFacade.mapAsList(((java.util.List)source.getListData()), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), ((ma.glasnost.orika.metadata.Type)usedTypes[3]), mappingContext));   
            destination.setListData(new_listData);   
        } else {  
            if ( !(((java.util.List)destination.getListData()) == null)) {  
                destination.setListData(null);  
            };  
        }if ( !(((java.util.Map)source.getMapData()) == null)){  
            java.util.Map new_mapData = ((java.util.Map)new java.util.LinkedHashMap());   
            for( java.util.Iterator mapData_$_iter = ((java.util.Map)source.getMapData()).entrySet().iterator(); mapData_$_iter.hasNext(); ) {   
                java.util.Map.Entry sourceMapDataEntry = ((java.util.Map.Entry)mapData_$_iter.next());   
                java.lang.Integer newMapDataKey = null;   
                java.util.List newMapDataVal = null;   
                if ( !(((java.lang.Long)sourceMapDataEntry.getKey()) == null)){   
                    newMapDataKey = ((java.lang.Integer)((ma.glasnost.orika.Converter)usedConverters[2]).convert(((java.lang.Long)sourceMapDataEntry.getKey()), ((ma.glasnost.orika.metadata.Type)usedTypes[3]), mappingContext));   
                } else {   
                    newMapDataKey = null;  
                }  
                if ( !(((java.util.List)sourceMapDataEntry.getValue()) == null)) {  
                    java.util.List new_newMapDataVal = ((java.util.List)new java.util.ArrayList());   
                    new_newMapDataVal.addAll(mapperFacade.mapAsList(((java.util.List)sourceMapDataEntry.getValue()), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), ((ma.glasnost.orika.metadata.Type)usedTypes[4]), mappingContext));   
                    newMapDataVal = new_newMapDataVal;   
                } else {  
                    if ( !(newMapDataVal == null)) {  
                        newMapDataVal = null;  
                    };  
                }  
                new_mapData.put(newMapDataKey, newMapDataVal);   
            }  
            destination.setMapData(new_mapData);   
        } else {  
            destination.setMapData(null);  
        }  
        destination.setP1(((java.lang.Integer)source.getP1()));   
        destination.setP2(((java.lang.Long)source.getP2()));   
        destination.setP3(((java.lang.Byte)source.getP3()));   
        destination.setPattr1(((java.lang.String)source.getPattr1()));   
        if ( !(((java.lang.String)source.getSeq()) == null)){   
            destination.setSeq(((java.lang.Long)((ma.glasnost.orika.Converter)usedConverters[3]).convert(((java.lang.String)source.getSeq()), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), mappingContext)));   
        } else {   
            destination.setSeq(null);  
        }  
        if(customMapper != null) {   
            customMapper.mapAtoB(source, destination, mappingContext);  
        }  
    }  
    public void mapBtoA(java.lang.Object a, java.lang.Object b, ma.glasnost.orika.MappingContext mappingContext) {  
        super.mapBtoA(a, b, mappingContext);  
        // sourceType: TargetVO  
        beanmapper_compare.vo.TargetVO source = ((beanmapper_compare.vo.TargetVO)a);   
        // destinationType: SourceVO  
        beanmapper_compare.vo.SourceVO destination = ((beanmapper_compare.vo.SourceVO)b);   
        destination.setFullName(((java.lang.String)source.getName()));   
        if ( !(((beanmapper_compare.vo.TargetVO.EnumType)source.getEnumType()) == null)){   
            destination.setType(((java.lang.Integer)((ma.glasnost.orika.Converter)usedConverters[0]).convert(((beanmapper_compare.vo.TargetVO.EnumType)source.getEnumType()), ((ma.glasnost.orika.metadata.Type)usedTypes[3]), mappingContext)));   
        } else {   
            destination.setType(null);  
        }  
        if ( !(((java.util.Date)source.getDate1()) == null)){   
            destination.setDate1(((java.util.Date)((ma.glasnost.orika.Converter)usedConverters[1]).convert(((java.util.Date)source.getDate1()), ((ma.glasnost.orika.metadata.Type)usedTypes[1]), mappingContext)));   
        } else {   
            destination.setDate1(null);  
        }if ( !(((java.util.List)source.getListData()) == null)) {  
            java.util.List new_listData = ((java.util.List)new java.util.ArrayList());   
            new_listData.addAll(mapperFacade.mapAsList(((java.util.List)source.getListData()), ((ma.glasnost.orika.metadata.Type)usedTypes[3]), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), mappingContext));   
            destination.setListData(new_listData);   
        } else {  
            if ( !(((java.util.List)destination.getListData()) == null)) {  
                destination.setListData(null);  
            };  
        }if ( !(((java.util.Map)source.getMapData()) == null)){  
            java.util.Map new_mapData = ((java.util.Map)new java.util.LinkedHashMap());   
            for( java.util.Iterator mapData_$_iter = ((java.util.Map)source.getMapData()).entrySet().iterator(); mapData_$_iter.hasNext(); ) {   
                java.util.Map.Entry sourceMapDataEntry = ((java.util.Map.Entry)mapData_$_iter.next());   
                java.lang.Long newMapDataKey = null;   
                java.util.List newMapDataVal = null;   
                if ( !(((java.lang.Integer)sourceMapDataEntry.getKey()) == null)){   
                    newMapDataKey = ((java.lang.Long)((ma.glasnost.orika.Converter)usedConverters[2]).convert(((java.lang.Integer)sourceMapDataEntry.getKey()), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), mappingContext));   
                } else {   
                    newMapDataKey = null;  
                }  
                if ( !(((java.util.List)sourceMapDataEntry.getValue()) == null)) {  
                    java.util.List new_newMapDataVal = ((java.util.List)new java.util.ArrayList());   
                    new_newMapDataVal.addAll(mapperFacade.mapAsList(((java.util.List)sourceMapDataEntry.getValue()), ((ma.glasnost.orika.metadata.Type)usedTypes[4]), ((ma.glasnost.orika.metadata.Type)usedTypes[2]), mappingContext));   
                    newMapDataVal = new_newMapDataVal;   
                } else {  
                    if ( !(newMapDataVal == null)) {  
                        newMapDataVal = null;  
                    };  
                }  
                new_mapData.put(newMapDataKey, newMapDataVal);   
            }  
            destination.setMapData(new_mapData);   
        } else {  
            destination.setMapData(null);  
        }  
        destination.setP1(((java.lang.Integer)source.getP1()));   
        destination.setP2(((java.lang.Long)source.getP2()));   
        destination.setP3(((java.lang.Byte)source.getP3()));   
        destination.setPattr1(((java.lang.String)source.getPattr1()));   
        if ( !(((java.lang.Long)source.getSeq()) == null)){   
            destination.setSeq(((java.lang.String)((ma.glasnost.orika.Converter)usedConverters[4]).convert(((java.lang.Long)source.getSeq()), ((ma.glasnost.orika.metadata.Type)usedTypes[5]), mappingContext)));   
        } else {   
            destination.setSeq(null);  
        }  
        if(customMapper != null) {   
            customMapper.mapBtoA(source, destination, mappingContext);  
        }  
    }  
}

这个mapper类就两个方法mapAtoB和mapBtoA，从名字看猜到前者是负责src -> dest的映射，后者是负责dest -> src的映射。
看看实现的过程。
Orika的使用跟Dozer的类似，首先通过配置生成一个MapperFactory，再用MapperFacade来作为映射的统一入口，这里MapperFactory和MapperFacade都是单例的。mapperFactory在做配置类映射时，只是注册了ClassMap，还没有真正的生成mapper的字节码，是在第一次调用getMapperFacade方法时才初始化mapper。下面看看getMapperFacade。
（源码基于 ma.glasnost.orika:orika-core:1.5.4）

public MapperFacade getMapperFacade() {  
    if (!isBuilt) {  
        synchronized (mapperFacade) {  
            if (!isBuilt) {  
                build();  
            }  
        }  
    }  
    return mapperFacade;  
}

利用注册的ClassMap信息和MappingContext上下文信息来构造mapper

public synchronized void build() {  
    if (!isBuilding && !isBuilt) {  
        isBuilding = true;  
        MappingContext context = contextFactory.getContext();  
        try {  
            if (useBuiltinConverters) {  
                BuiltinConverters.register(converterFactory);  
            }  
            converterFactory.setMapperFacade(mapperFacade);  
            for (Map.Entry<MapperKey, ClassMap<Object, Object>> classMapEntry : classMapRegistry.entrySet()) {  
                ClassMap<Object, Object> classMap = classMapEntry.getValue();  
                if (classMap.getUsedMappers().isEmpty()) {  
                    classMapEntry.setValue(classMap.copyWithUsedMappers(discoverUsedMappers(classMap)));  
                }  
            }  
            buildClassMapRegistry();  
            Map<ClassMap<?, ?>, GeneratedMapperBase> generatedMappers = new HashMap<ClassMap<?, ?>, GeneratedMapperBase>();  
            //重点看这里  
            //在使用mapperFactory配置classMap时，会存放在classMapRegistry里  
            for (ClassMap<?, ?> classMap : classMapRegistry.values()) {  
                //对每个classMap生成一个mapper，重点看buildMapper方法  
                generatedMappers.put(classMap, buildMapper(classMap, false, context));  
            }  
            Set<Entry<ClassMap<?, ?>, GeneratedMapperBase>> generatedMapperEntries = generatedMappers.entrySet();  
            for (Entry<ClassMap<?, ?>, GeneratedMapperBase> generatedMapperEntry : generatedMapperEntries) {  
                buildObjectFactories(generatedMapperEntry.getKey(), context);  
                initializeUsedMappers(generatedMapperEntry.getValue(), generatedMapperEntry.getKey(), context);  
            }  
        } finally {  
            contextFactory.release(context);  
        }  
        isBuilt = true;  
        isBuilding = false;  
    }  
}  
public Set<ClassMap<Object, Object>> lookupUsedClassMap(MapperKey mapperKey) {  
    Set<ClassMap<Object, Object>> usedClassMapSet = usedMapperMetadataRegistry.get(mapperKey);  
    if (usedClassMapSet == null) {  
        usedClassMapSet = Collections.emptySet();  
    }  
    return usedClassMapSet;  
}

跟踪buildMapper方法

private GeneratedMapperBase buildMapper(ClassMap<?, ?> classMap, boolean isAutoGenerated, MappingContext context) {  
    register(classMap.getAType(), classMap.getBType(), isAutoGenerated);  
    register(classMap.getBType(), classMap.getAType(), isAutoGenerated);  
    final MapperKey mapperKey = new MapperKey(classMap.getAType(), classMap.getBType());  
    //调用mapperGenerator的build方法生成mapper  
    final GeneratedMapperBase mapper = mapperGenerator.build(classMap, context);  
    mapper.setMapperFacade(mapperFacade);  
    mapper.setFromAutoMapping(isAutoGenerated);  
    if (classMap.getCustomizedMapper() != null) {  
        final Mapper<Object, Object> customizedMapper = (Mapper<Object, Object>) classMap.getCustomizedMapper();  
        mapper.setCustomMapper(customizedMapper);  
    }  
    mappersRegistry.remove(mapper);  
    //生成的mapper存放到mappersRegistry  
    mappersRegistry.add(mapper);  
    classMapRegistry.put(mapperKey, (ClassMap<Object, Object>) classMap);  
    return mapper;  
}
MapperGenerator的build方法
public GeneratedMapperBase build(ClassMap<?, ?> classMap, MappingContext context) {  
StringBuilder logDetails = null;  
try {  
    compilerStrategy.assureTypeIsAccessible(classMap.getAType().getRawType());  
    compilerStrategy.assureTypeIsAccessible(classMap.getBType().getRawType());  
    if (LOGGER.isDebugEnabled()) {  
        logDetails = new StringBuilder();  
        String srcName = TypeFactory.nameOf(classMap.getAType(), classMap.getBType());  
        String dstName = TypeFactory.nameOf(classMap.getBType(), classMap.getAType());  
        logDetails.append("Generating new mapper for (" + srcName + ", " + dstName + ")");  
    }  
    //构建用来生成源码及字节码的上下文  
    final SourceCodeContext mapperCode = new SourceCodeContext(classMap.getMapperClassName(), GeneratedMapperBase.class, context,  
            logDetails);  
    Set<FieldMap> mappedFields = new LinkedHashSet<FieldMap>();  
    //增加mapAtoB方法  
    mappedFields.addAll(addMapMethod(mapperCode, true, classMap, logDetails));  
    //增加mapBtoA方法  
    //addMapMethod方法基本就是手写代码的过程，有兴趣的读者可以看看  
    mappedFields.addAll(addMapMethod(mapperCode, false, classMap, logDetails));  
    //生成一个mapper实例  
    GeneratedMapperBase instance = mapperCode.getInstance();  
    instance.setAType(classMap.getAType());  
    instance.setBType(classMap.getBType());  
    instance.setFavorsExtension(classMap.favorsExtension());  
    if (logDetails != null) {  
        LOGGER.debug(logDetails.toString());  
        logDetails = null;  
    }  
    classMap = classMap.copy(mappedFields);  
    context.registerMapperGeneration(classMap);  
    return instance;  
} catch (final Exception e) {  
    if (logDetails != null) {  
        logDetails.append("\n<---- ERROR occurred here");  
        LOGGER.debug(logDetails.toString());  
    }  
    throw new MappingException(e);  
}

生成mapper实例

T instance = (T) compileClass().newInstance();  
protected Class<?> compileClass() throws SourceCodeGenerationException {  
    try {  
        return compilerStrategy.compileClass(this);  
    } catch (SourceCodeGenerationException e) {  
        throw e;  
    }  
}

这里的compilerStrategy的默认是用Javassist（也可以自定义生成字节码的策略）
JavassistCompilerStrategy的compileClass方法
这基本上就是一个使用Javassist的过程，经过前面的各种铺垫（通过配置信息、上下文信息、拼装java源代码等等），终于来到这一步

public Class<?> compileClass(SourceCodeContext sourceCode) throws SourceCodeGenerationException {  
    StringBuilder className = new StringBuilder(sourceCode.getClassName());  
    CtClass byteCodeClass = null;  
    int attempts = 0;  
    Random rand = RANDOM;  
    while (byteCodeClass == null) {  
        try {  
            //创建一个类  
            byteCodeClass = classPool.makeClass(className.toString());  
        } catch (RuntimeException e) {  
            if (attempts < 5) {  
                className.append(Integer.toHexString(rand.nextInt()));  
            } else {  
                // No longer likely to be accidental name collision;  
                // propagate the error  
                throw e;  
            }  
        }  
    }  
    CtClass abstractMapperClass;  
    Class<?> compiledClass;  
    try {  
        //把源码写到磁盘（通过上面提到的配置）  
        writeSourceFile(sourceCode);  
        Boolean existing = superClasses.put(sourceCode.getSuperClass(), true);  
        if (existing == null || !existing) {  
            classPool.insertClassPath(new ClassClassPath(sourceCode.getSuperClass()));  
        }  
        if (registerClassLoader(Thread.currentThread().getContextClassLoader())) {  
            classPool.insertClassPath(new LoaderClassPath(Thread.currentThread().getContextClassLoader()));  
        }  
        abstractMapperClass = classPool.get(sourceCode.getSuperClass().getCanonicalName());  
        byteCodeClass.setSuperclass(abstractMapperClass);  
        //增加字段  
        for (String fieldDef : sourceCode.getFields()) {  
            try {  
                byteCodeClass.addField(CtField.make(fieldDef, byteCodeClass));  
            } catch (CannotCompileException e) {  
                LOG.error("An exception occurred while compiling: " + fieldDef + " for " + sourceCode.getClassName(), e);  
                throw e;  
            }  
        }  
        //增加方法，这里主要就是mapAtoB和mapBtoA方法  
        //直接用源码通过Javassist往类“加”方法  
        for (String methodDef : sourceCode.getMethods()) {  
            try {  
                byteCodeClass.addMethod(CtNewMethod.make(methodDef, byteCodeClass));  
            } catch (CannotCompileException e) {  
                LOG.error(  
                    "An exception occured while compiling the following method:\n\n " + methodDef + "\n\n for "  
                    + sourceCode.getClassName() + "\n", e);  
                throw e;  
            }  
        }  
        //生成类  
        compiledClass = byteCodeClass.toClass(Thread.currentThread().getContextClassLoader(), this.getClass().getProtectionDomain());  
        //字节码文件写磁盘  
        writeClassFile(sourceCode, byteCodeClass);  
    } catch (NotFoundException e) {  
        throw new SourceCodeGenerationException(e);  
    } catch (CannotCompileException e) {  
        throw new SourceCodeGenerationException("Error compiling " + sourceCode.getClassName(), e);  
    } catch (IOException e) {  
        throw new SourceCodeGenerationException("Could not write files for " + sourceCode.getClassName(), e);  
    }  
    return compiledClass;  
}

mapper类生成了，现在就看在调用MapperFacade的map方法是如何使用这个mapper类的。
其实很简单，还记得生成的mapper是放到mappersRegistry吗，跟踪代码，在resolveMappingStrategy方法根据typeA和typeB在mappersRegistry找到mapper，在调用mapper的mapAtoB或mapBtoA方法即可。

小结

总体来说，Orika是一个功能强大的而且性能很高的工具，推荐使用。

总结

通过对BeanUtils、BeanCopier、Dozer、Orika这几个工具的对比，得知了它们的性能以及实现原理。在使用时，可以根据自己的实际情况选择，推荐使用Orika。

BeanUtils、BeanCopier、Dozer、Orika性能比较

背景

性能分析

Spring的BeanUtils

使用

原理

小结

Apache的BeanUtils

使用

原理

小结

BeanCopier

使用

原理

小结

Dozer

使用

原理

小结

Orika

使用

原理

小结

总结

Spring的`BeanUtils`

Apache的`BeanUtils`

`BeanCopier`

`Dozer`

`Orika`