类图

image.png

代理模式

静态代理实现

PrintaPrintableble

  1. public interface Printable {
  2.     void setPrinterName(String name);
  4.     String getPrinterName();
  6.     void print(String str);
  7. }

Printer

  1. import java.util.concurrent.TimeUnit;
  3. public class Printer implements Printable {
  4.     private String name;
  6.     public Printer(String name) {
  7.         this.name = name;
  8.         heavyJob("正在生成Printer的实例(" + name + ")");
  9.     }
  11.     public Printer() {
  12.         heavyJob("正在生成Printer的实例(" + name + ")");
  13.     }
  15.     @Override
  16.     public void setPrinterName(String name) {
  17.         this.name = name;
  18.     }
  20.     @Override
  21.     public String getPrinterName() {
  22.         return name;
  23.     }
  25.     @Override
  26.     public void print(String str) {
  27.         System.out.println("======" + name + "=====");
  28.         System.out.println(str);
  29.     }
  31.     private void heavyJob(String msg) {
  32.         System.out.println(msg);
  33.         for (int i = 0; i < 5; i++) {
  34.             try {
  35.                 TimeUnit.SECONDS.sleep(1);
  36.             } catch (InterruptedException e) {
  37.                 e.printStackTrace();
  38.             }
  39.             System.out.print(".");
  40.         }
  41.         System.out.println("结束");
  42.     }
  43. }

PrinterProxy

  1. public class PrinterProxy implements Printable {
  2.     private String name;
  3.     private Printer real;
  5.     public PrinterProxy() {
  6.     }
  8.     public PrinterProxy(String name) {
  9.         this.name = name;
  10.     }
  12.     @Override
  13.     public void setPrinterName(String name) {
  14.         if (real != null) {
  15.             real.setPrinterName(name);
  16.         }
  17.         this.name = name;
  18.     }
  20.     @Override
  21.     public String getPrinterName() {
  22.         return name;
  23.     }
  25.     @Override
  26.     public void print(String str) {
  27.         realize();
  28.         real.print(str);
  29.     }
  31.     private void realize() {
  32.         if (real == null) {
  33.             real = new Printer(name);
  34.         }
  35.     }
  36. }

Printe>rPrPrinterProxyoxy

Main

  1. public class Main {
  2.     public static void main(String[] args) {
  3.         Printable p = new PrinterProxy("Alice");
  4.         System.out.println("现在的名字是："+p.getPrinterName());
  6.         p.setPrinterName("Bob");
  7.         System.out.println("现在的名字是："+p.getPrinterName());
  8.         p.print("Hello, world.");
  9.     }
  10. }

类图，如下所示：

Main.png

不论setPrinterName方法和getPrinterName方法被调用多少次，都不会生成Printer类的实例。Printer类并不知道PrinterProxy类的存在，即Printer类并不知道自己到底是通过PrinterProxy被调用的还是直接被调用的。

动态代理

JDK动态代理

依据运行时托自省能力，在运行时动态获取类或者对象的信息

利用动态代理辅助打印 jdk二分搜索的执行轨迹

  1. package com.cninfo.designpattern.structural.proxy.repeat02;
  3. import java.lang.reflect.InvocationHandler;
  4. import java.lang.reflect.Method;
  5. import java.lang.reflect.Proxy;
  6. import java.util.Arrays;
  7. import java.util.concurrent.CountDownLatch;
  9. /**
  10.  * @author chenxinwei
  11.  * @date 2021/5/13 17:11
  12.  **/
  13. public class ProxyMain {
  14.     private static final int COUNT = 1000;
  16.     public static void main(String[] args) {
  17.         Comparable[] values = new Comparable[COUNT];
  19.         for (int i = 0; i < COUNT; i++) {
  20.             Integer value = Integer.valueOf(i);
  21.             values[i] = (Comparable) Proxy.newProxyInstance(
  22.                     null,
  23.                     value.getClass().getInterfaces(),
  24.                     new InvocationHandler() {
  25.                         @Override
  26.                         public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  27.                             System.out.println(value + "." + method.getName() + Arrays.toString(args));
  28.                             return method.invoke(value, args);
  29.                         }
  30.                     });
  31.         }
  33.         CountDownLatch countDownLatch = new CountDownLatch(1);
  35.         System.out.println(Arrays.binarySearch(values, 264));
  36.         try {
  37.             countDownLatch.await();
  38.         } catch (InterruptedException e) {
  39.             e.printStackTrace();
  40.         }
  41.     }
  42. }
  1. 499.compareTo[264]
  2. 249.compareTo[264]
  3. 374.compareTo[264]
  4. 311.compareTo[264]
  5. 280.compareTo[264]
  6. 264.compareTo[264]
  7. 264

查看生成的代理类

使用arthas-boot.jar，下载地址参考：https://arthas.aliyun.com/doc/download.html

  1. 搜索包下的class, sc com.cninfo.designpattern.structural.proxy.repeat02.*
  2. 反编译class jad com.cninfo.designpattern.structural.proxy.repeat02.ProxyMain$1

image.png

字节码动态代理

asm(cglib)

Proxy生成的代理类时，对原始类有要求（该类必须实现接口）。如果给一个没有实现接口的类生成代理，可以使用cglib
用来对class进行增强

cglib is a powerful, high performance and quality Code Generation Library. It is used to extend JAVA classes and implements interfaces at runtime.

  • cglib-#.#_#.jar
    • binary distribution, CGLIB classes only, it must be used to extend cglib classes dependant on ASM API
  • cglib-nodep-#.#_#.jar
    • binary distribution, CGLIB and renamed ASM classes, not extendable

分为依赖ASM和不依赖ASM的两种实现

MyBean

  1. package com.cninfo.designpattern.structural.proxy.cglib;
  3. import java.beans.PropertyChangeListener;
  4. import java.io.Serializable;
  6. /**
  7.  * @author chenxinwei
  8.  * @date 2021/5/13 18:15
  9.  **/
  10. public abstract class MyBean implements Serializable {
  11.     String property;
  13.     abstract public void addPropertyChangeListener(PropertyChangeListener propertyChangeListener);
  15.     abstract public void removePropertyChangeListener(PropertyChangeListener propertyChangeListener);
  17.     public String getProperty() {
  18.         return property;
  19.     }
  21.     public void setProperty(String property) {
  22.         this.property = property;
  23.     }
  25.     @Override
  26.     public String toString() {
  27.         return "MyBean{" +
  28.                 "property='" + property + '\'' +
  29.                 '}';
  30.     }
  31. }

MyBeanEnhancer

  1. package com.cninfo.designpattern.structural.proxy.cglib;
  3. import net.sf.cglib.proxy.Enhancer;
  4. import net.sf.cglib.proxy.MethodInterceptor;
  5. import net.sf.cglib.proxy.MethodProxy;
  7. import java.beans.PropertyChangeListener;
  8. import java.beans.PropertyChangeSupport;
  9. import java.lang.reflect.Method;
  10. import java.lang.reflect.Modifier;
  11. import java.util.concurrent.CountDownLatch;
  13. /**
  14.  * @author chenxinwei
  15.  * @date 2021/5/13 18:17
  16.  **/
  17. public class MyBeanEnhancer implements MethodInterceptor {
  18.     private PropertyChangeSupport propertyChangeSupport;
  20.     public void addPropertyChangeListener(PropertyChangeListener listener) {
  21.         propertyChangeSupport.addPropertyChangeListener(listener);
  22.     }
  24.     public void removePropertyChangeListener(PropertyChangeListener listener) {
  25.         propertyChangeSupport.removePropertyChangeListener(listener);
  26.     }
  28.     public static Object newInstance(Class clazz) {
  29.         MyBeanEnhancer interceptor = new MyBeanEnhancer();
  30.         Enhancer enhancer = new Enhancer();
  31.         enhancer.setSuperclass(clazz);
  32.         enhancer.setCallback(interceptor);
  34.         Object bean = enhancer.create();
  35.         interceptor.propertyChangeSupport = new PropertyChangeSupport(bean);
  36.         return bean;
  37.     }
  39.     @Override
  40.     public Object intercept(Object o, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
  41.         Object retValFormSuper = null;
  42.         try {
  43.             if (!Modifier.isAbstract(method.getModifiers())) {
  44.                 retValFormSuper = methodProxy.invokeSuper(o, args);
  45.             }
  46.         } finally {
  47.             String name = method.getName();
  48.             if (name.equals("addPropertyChangeListener")) {
  49.                 addPropertyChangeListener((PropertyChangeListener) args[0]);
  50.             } else if (name.equals("removePropertyChangeListener")) {
  51.                 removePropertyChangeListener((PropertyChangeListener) args[0]);
  52.             }
  53.             if (name.startsWith("set") &&
  54.                     args.length == 1 &&
  55.                     method.getReturnType() == Void.TYPE) {
  56.                 char propName[] = name.substring("set".length()).toCharArray();
  58.                 propName[0] = Character.toLowerCase(propName[0]);
  59.                 propertyChangeSupport.firePropertyChange(new String(propName), null, args[0]);
  61.             }
  62.         }
  63.         return retValFormSuper;
  64.     }
  66.     public static void main(String[] args) {
  67.         MyBean bean = (MyBean) newInstance(MyBean.class);
  68.         bean.addPropertyChangeListener(System.out::println);
  69.         bean.setProperty("TEST");
  71.         CountDownLatch countDownLatch = new CountDownLatch(1);
  72.         try {
  73.             countDownLatch.await();
  74.         } catch (InterruptedException e) {
  75.             e.printStackTrace();
  76.         }
  77.     }
  78. }

输出：

  1. java.beans.PropertyChangeEvent[propertyName=property; oldValue=null; newValue=TEST; propagationId=null; source=MyBean{property='TEST'}]

javassist

了解
Javassist是一个开源的分析、编辑和创建Java字节码的类库，可以直接编辑和生成Java生成的字节码。相对于bcel, asm等这些工具，开发者不需要了解虚拟机指令，就能动态改变类的结构，或者动态生成类。javassist简单易用， 快速。

JDK动态代理和CGLIB动态代理

  1. JDK动态代理只提供接口的代理，不支持类的代理。
  2. CGLIB 是一个代码生成类库，可以在运行时动态生成指定类的一个子类代理对象，并覆盖其中特定方法并添加增强代码，从而实现AOP。CGLIB是通过继承的方式做动态代理，因此，如果某个类被标记为final，那么它无法使用CGLIG做动态代理
  3. 静态代理与动态代理的区别在于生成AOP对象的时机不同，相对来说AspectJ的静态代理方式具有更好的性能，但是AspectJ需要特定的编译器进行处理。而Spring AOP则无需特定的编译器处理。

直接使用ASM，要求你必须对JVM内部结构包括class文件的格式和指令集都很熟悉。所以使用cglib是多数人的选择（够用即可）。

MyBatis为接口生成代理类的过程

代理模式 - 图4

代理模式 - 图5

  1. 初始化的时候最终会调到 org.apache.ibatis.binding.MapperRegistry#``addMapper方法
  2. addMapper方法将new MapperProxyFactory<>(type) 存入到MapperRegistry的变量knowMappers中
  3. new MapperProxyFactory<>(type)负责为type接口生成代理类，具体参见 org.apache.ibatis.binding.MapperProxyFactory#newInstance(org.apache.ibatis.session.SqlSession)
  4. newInstance中，使用实现了InvocationHandler接口的MapperProxy类，负责具体处理，org.apache.ibatis.binding.MapperProxy#invoke是代理逻辑的核心所在。最终会调用mapperMethod``.execute(``sqlSession``**, **``args)方法。

mapperMethod``.execute(``sqlSession``**, **``args)方法，最终会将交由SqlSession执行具体的数据库操作

  1.  public Object execute(SqlSession sqlSession, Object[] args) {
  2.     Object result;
  3.     switch (command.getType()) {
  4.       case INSERT: {
  5.         Object param = method.convertArgsToSqlCommandParam(args);
  6.         result = rowCountResult(sqlSession.insert(command.getName(), param));
  7.         break;
  8.       }
  9.       case UPDATE: {
  10.         Object param = method.convertArgsToSqlCommandParam(args);
  11.         result = rowCountResult(sqlSession.update(command.getName(), param));
  12.         break;
  13.       }
  14.       case DELETE: {
  15.         Object param = method.convertArgsToSqlCommandParam(args);
  16.         result = rowCountResult(sqlSession.delete(command.getName(), param));
  17.         break;
  18.       }
  19.       case SELECT:
  20.         if (method.returnsVoid() && method.hasResultHandler()) {
  21.           executeWithResultHandler(sqlSession, args);
  22.           result = null;
  23.         } else if (method.returnsMany()) {
  24.           result = executeForMany(sqlSession, args);
  25.         } else if (method.returnsMap()) {
  26.           result = executeForMap(sqlSession, args);
  27.         } else if (method.returnsCursor()) {
  28.           result = executeForCursor(sqlSession, args);
  29.         } else {
  30.           Object param = method.convertArgsToSqlCommandParam(args);
  31.           result = sqlSession.selectOne(command.getName(), param);
  32.           if (method.returnsOptional()
  33.               && (result == null || !method.getReturnType().equals(result.getClass()))) {
  34.             result = Optional.ofNullable(result);
  35.           }
  36.         }
  37.         break;
  38.       case FLUSH:
  39.         result = sqlSession.flushStatements();
  40.         break;
  41.       default:
  42.         throw new BindingException("Unknown execution method for: " + command.getName());
  43.     }
  44.     if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
  45.       throw new BindingException("Mapper method '" + command.getName()
  46.           + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
  47.     }
  48.     return result;
  49.   }
  50. // case SELECT方法中的 method.returnsMany()
  51.   private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
  52.     List<E> result;
  53.     Object param = method.convertArgsToSqlCommandParam(args);
  54.     if (method.hasRowBounds()) {
  55.       RowBounds rowBounds = method.extractRowBounds(args);
  56.       result = sqlSession.selectList(command.getName(), param, rowBounds);
  57.     } else {
  58.       result = sqlSession.selectList(command.getName(), param);
  59.     }
  60.     // issue #510 Collections & arrays support
  61.     if (!method.getReturnType().isAssignableFrom(result.getClass())) {
  62.       if (method.getReturnType().isArray()) {
  63.         return convertToArray(result);
  64.       } else {
  65.         return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
  66.       }
  67.     }
  68.     return result;
  69.   }
  1. public class MapperMethod {
  3.   private final SqlCommand command;
  4.   private final MethodSignature method;
  5.  //   ...
  6.  }
  8.  public static class SqlCommand {
  10.     private final String name; // 接口全限名
  11.     private final SqlCommandType type; //sql类型 UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH
  13.      //   ...
  14.  }
  16.   public static class MethodSignature {
  18.     private final boolean returnsMany;
  19.     private final boolean returnsMap;
  20.     private final boolean returnsVoid;
  21.     private final boolean returnsCursor;
  22.     private final boolean returnsOptional;
  23.     private final Class<?> returnType;
  24.     private final String mapKey;
  25.     private final Integer resultHandlerIndex;
  26.     private final Integer rowBoundsIndex;
  27.     private final ParamNameResolver paramNameResolver;
  29.      //   ...
  30.  }