实用篇 - MyBatis 示例-插件 - 《Java 开发笔记》

简介
插件开发过程

简介

利用 MyBatis Plugin 插件技术实现分页功能。

分页插件实现思路如下：

业务代码在 ThreadLocal 中保存分页信息；
MyBatis Interceptor 拦截查询请求，获取分页信息，实现分页操作，封装分页列表数据返回；

测试类：com.yjw.demo.PageTest

插件开发过程

确定需要拦截的签名

MyBatis 插件可以拦截四大对象中的任意一个，从 Plugin 源码中可以看到它需要注册签名才能够运行插件，签名需要确定一些要素。

确定需要拦截的对象

Executor 是执行 SQL 的全过程，包括组装参数，组装结果集返回和执行 SQL 过程，都可以拦截。
StatementHandler 是执行 SQL 的过程，我们可以重写执行 SQL 的过程。
ParameterHandler 是拦截执行 SQL 的参数组装，我们可以重写组装参数规则。
ResultSetHandler 用于拦截执行结果的组装，我们可以重写组装结果的规则。

拦截方法和参数

当确定了需要拦截什么对象，接下来就要确定需要拦截什么方法和方法的参数。比如分页插件需要拦截 Executor 的 query 方法，我们先看看 Executor 接口的定义，代码清单如下：

public interface Executor {
  ResultHandler NO_RESULT_HANDLER = null;
  int update(MappedStatement ms, Object parameter) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
  <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
  <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
  List<BatchResult> flushStatements() throws SQLException;
  void commit(boolean required) throws SQLException;
  void rollback(boolean required) throws SQLException;
  CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
  boolean isCached(MappedStatement ms, CacheKey key);
  void clearLocalCache();
  void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
  Transaction getTransaction();
  void close(boolean forceRollback);
  boolean isClosed();
  void setExecutorWrapper(Executor executor);
}

以上的任何方法都可以拦截，从接口定义而言，query 方法有两个，我们可以按照代码清单来定义签名。

@Intercepts({
        @Signature(type = Executor.class, method = "query",
                args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
        @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class,
                ResultHandler.class, CacheKey.class, BoundSql.class})})

其中，@Intercepts 说明它是一个拦截器。@Signature 是注册拦截器签名的地方，type 是四大对象中的一个，method 是需要拦截的方法，args 是方法的参数。

插件接口定义

在 MyBatis 中开发插件，需要实现 Interceptor 接口，接口的定义如下：

public interface Interceptor {
  Object intercept(Invocation invocation) throws Throwable;
  Object plugin(Object target);
  void setProperties(Properties properties);
}

intercept 方法：它将直接覆盖你所拦截对象原有的方法，因此它是插件的核心方法。通过 invocation 参数可以反射调度原来对象的方法。
plugin 方法：target 是被拦截对象，它的作用是给被拦截对象生成一个代理对象，并返回它。为了方便 MyBatis 使用 org.apache.ibatis.plugin.Plugin 中的 wrap 静态方法提供生成代理对象。
setProperties 方法：允许在 plugin 元素中配置所需参数，方法在插件初始化的时候就被调用了一次，然后把插件对象存入到配置中，以便后面再取出。

实现类

根据分页插件的实现思路，定义了三个类。

Page 类

Page 类继承了 ArrayList 类，用来封装分页信息和列表数据。

/**
 * 分页返回对象
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
public class Page<E> extends ArrayList<E> {
    private static final long serialVersionUID = 1L;
    /**
     * 页码，从1开始
     */
    private int pageNum;
    /**
     * 页面大小
     */
    private int pageSize;
    /**
     * 起始行
     */
    private int startRow;
    /**
     * 末行
     */
    private int endRow;
    /**
     * 总数
     */
    private long total;
    /**
     * 总页数
     */
    private int pages;
    public int getPageNum() {
        return pageNum;
    }
    public void setPageNum(int pageNum) {
        this.pageNum = pageNum;
    }
    public int getPageSize() {
        return pageSize;
    }
    public void setPageSize(int pageSize) {
        this.pageSize = pageSize;
    }
    public int getStartRow() {
        return startRow;
    }
    public void setStartRow(int startRow) {
        this.startRow = startRow;
    }
    public int getEndRow() {
        return endRow;
    }
    public void setEndRow(int endRow) {
        this.endRow = endRow;
    }
    public long getTotal() {
        return total;
    }
    public void setTotal(long total) {
        this.total = total;
        this.pages = (int)(total / pageSize + (total % pageSize == 0 ? 0 : 1));
        if (pageNum > pages) {
            pageNum = pages;
        }
        this.startRow = this.pageNum > 0 ? (this.pageNum - 1) * this.pageSize : 0;
        this.endRow = this.startRow + this.pageSize * (this.pageNum > 0 ? 1 : 0);
    }
    public int getPages() {
        return pages;
    }
    public void setPages(int pages) {
        this.pages = pages;
    }
    /**
     * 返回当前对象
     * 
     * @return
     */
    public List<E> getResult() {
        return this;
    }
}

PageHelper 类

PageHelper 类是分页的帮助类，主要利用 ThreadLocal 线程变量存储分页信息。代码清单如下：

/**
 * 分页帮助类
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
@SuppressWarnings("rawtypes")
public class PageHelper {
    private static final ThreadLocal<Page> PAGE_THREADLOCAT = new ThreadLocal<Page>();
    /**
     * 设置线程局部变量分页信息
     * 
     * @param page
     */
    public static void setPageThreadLocal(Page page) {
        PAGE_THREADLOCAT.set(page);
    }
    /**
     * 获取线程局部变量分页信息
     * 
     * @return
     */
    public static Page getPageThreadLocal() {
        return PAGE_THREADLOCAT.get();
    }
    /**
     * 清空线程局部变量分页信息
     */
    public static void pageThreadLocalClear() {
        PAGE_THREADLOCAT.remove();
    }
    /**
     * 设置分页参数
     * 
     * @param pageNum
     * @param pageSize
     */
    public static void startPage(Integer pageNum, Integer pageSize) {
        Page page = new Page();
        page.setPageNum(pageNum);
        page.setPageSize(pageSize);
        setPageThreadLocal(page);
    }
}

PageInterceptor 类

PageInterceptor 类实现了 Interceptor 接口，是分页插件的核心类。代码清单如下：

/**
 * 分页拦截器
 * 
 * @author yinjianwei
 * @date 2018/11/05
 */
@Intercepts({
    @Signature(type = Executor.class, method = "query",
        args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
    @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class,
        ResultHandler.class, CacheKey.class, BoundSql.class})})
public class PageInterceptor implements Interceptor {
    private Field additionalParametersField;
    @SuppressWarnings({"rawtypes", "unchecked"})
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        Executor executor = (Executor)invocation.getTarget();
        Object[] args = invocation.getArgs();
        MappedStatement ms = (MappedStatement)args[0];
        Object parameter = args[1];
        RowBounds rowBounds = (RowBounds)args[2];
        ResultHandler resultHandler = (ResultHandler)args[3];
        CacheKey cacheKey;
        BoundSql boundSql;
        // 4个参数
        if (args.length == 4) {
            boundSql = ms.getBoundSql(parameter);
            cacheKey = executor.createCacheKey(ms, parameter, rowBounds, boundSql);
        }
        // 6个参数
        else {
            cacheKey = (CacheKey)args[4];
            boundSql = (BoundSql)args[5];
        }
        // 判断是否需要分页
        Page page = PageHelper.getPageThreadLocal();
        // 不执行分页
        if (page.getPageNum() <= 0) {
            return executor.query(ms, parameter, rowBounds, resultHandler, cacheKey, boundSql);
        }
        // count查询
        MappedStatement countMs = newCountMappedStatement(ms);
        String sql = boundSql.getSql();
        String countSql = "select count(1) from (" + sql + ") _count";
        BoundSql countBoundSql =
            new BoundSql(ms.getConfiguration(), countSql, boundSql.getParameterMappings(), parameter);
        Map<String, Object> additionalParameters = (Map<String, Object>)additionalParametersField.get(boundSql);
        for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) {
            countBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue());
        }
        CacheKey countCacheKey = executor.createCacheKey(countMs, parameter, rowBounds, countBoundSql);
        Object countResult =
            executor.query(countMs, parameter, RowBounds.DEFAULT, resultHandler, countCacheKey, countBoundSql);
        Long count = (Long)((List)countResult).get(0);
        page.setTotal(count);
        // 分页查询
        String pageSql = sql + " limit " + page.getStartRow() + "," + page.getPageSize();
        BoundSql pageBoundSql =
            new BoundSql(ms.getConfiguration(), pageSql, boundSql.getParameterMappings(), parameter);
        for (Entry<String, Object> additionalParameter : additionalParameters.entrySet()) {
            pageBoundSql.setAdditionalParameter(additionalParameter.getKey(), additionalParameter.getValue());
        }
        CacheKey pageCacheKey = executor.createCacheKey(ms, parameter, rowBounds, pageBoundSql);
        List listResult = executor.query(ms, parameter, RowBounds.DEFAULT, resultHandler, pageCacheKey, pageBoundSql);
        page.addAll(listResult);
        // 清空线程局部变量分页信息
        PageHelper.pageThreadLocalClear();
        return page;
    }
    @Override
    public Object plugin(Object target) {
        return Plugin.wrap(target, this);
    }
    @Override
    public void setProperties(Properties properties) {
        try {
            additionalParametersField = BoundSql.class.getDeclaredField("additionalParameters");
            additionalParametersField.setAccessible(true);
        } catch (NoSuchFieldException | SecurityException e) {
            e.printStackTrace();
        }
    }
    /**
     * 创建count的MappedStatement
     * 
     * @param ms
     * @return
     */
    private MappedStatement newCountMappedStatement(MappedStatement ms) {
        MappedStatement.Builder builder = new MappedStatement.Builder(ms.getConfiguration(), ms.getId() + "_count",
            ms.getSqlSource(), ms.getSqlCommandType());
        builder.resource(ms.getResource());
        builder.fetchSize(ms.getFetchSize());
        builder.statementType(ms.getStatementType());
        builder.keyGenerator(ms.getKeyGenerator());
        if (ms.getKeyProperties() != null && ms.getKeyProperties().length != 0) {
            StringBuilder keyProperties = new StringBuilder();
            for (String keyProperty : ms.getKeyProperties()) {
                keyProperties.append(keyProperty).append(",");
            }
            keyProperties.delete(keyProperties.length() - 1, keyProperties.length());
            builder.keyProperty(keyProperties.toString());
        }
        builder.timeout(ms.getTimeout());
        builder.parameterMap(ms.getParameterMap());
        // count查询返回值int
        List<ResultMap> resultMaps = new ArrayList<ResultMap>();
        ResultMap resultMap = new ResultMap.Builder(ms.getConfiguration(), ms.getId() + "_count", Long.class,
            new ArrayList<ResultMapping>(0)).build();
        resultMaps.add(resultMap);
        builder.resultMaps(resultMaps);
        builder.resultSetType(ms.getResultSetType());
        builder.cache(ms.getCache());
        builder.flushCacheRequired(ms.isFlushCacheRequired());
        builder.useCache(ms.isUseCache());
        return builder.build();
    }
}

配置

MyBatis 配置文件增加 plugin 配置项。

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>
    <typeHandlers>
        <typeHandler javaType="com.yjw.demo.mybatis.common.constant.Sex"
                     jdbcType="TINYINT"
                     handler="com.yjw.demo.mybatis.common.type.SexEnumTypeHandler"/>
    </typeHandlers>
    <plugins>
        <plugin interceptor="com.yjw.demo.mybatis.common.page.PageInterceptor">
        </plugin>
    </plugins>
</configuration>

作者：殷建卫链接：https://www.yuque.com/yinjianwei/vyrvkf/qh6cfu 来源：殷建卫 - 架构笔记著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。