系列

• MyBatis拦截器原理介绍
• Mybatis拦截器改写请求参数和结果
• Mybatis 插件兼容动态SQL
• mybatis 参数解析流程附相关案例
• 再谈mybatis执行流程

Mybatis拦截器介绍

MyBatis 允许你在映射语句执行过程中的某一点进行拦截调用。默认情况下，MyBatis 允许使用插件来拦截的方法调用包括：

• Executor (update, query, flushStatements, commit, rollback, getTransaction, close, isClosed) 拦截执行器的方法
• ParameterHandler (getParameterObject, setParameters) 拦截参数的处理
• ResultSetHandler (handleResultSets, handleOutputParameters) 拦截结果集的处理
• StatementHandler (prepare, parameterize, batch, update, query) 拦截Sql语法和会话构建的处理

mybatis执行流程

• mybatis在执行过程中按照Executor => StatementHandler => ParameterHandler => ResultSetHandler。
• Executor在执行过程中会创建StatementHandler，在创建StatementHandler过程中会创建 ParameterHandler和ResultSetHandler。

Mybatis拦截器的使用

---------定义拦截器
@Intercepts({
        @Signature(type = ParameterHandler.class, method = "setParameters", args = PreparedStatement.class),
        @Signature(type = Executor.class, method = "update", args = {MappedStatement.class, Object.class}),
        @Signature(type = Executor.class, method = "createCacheKey", args = {MappedStatement.class, Object.class, RowBounds.class, BoundSql.class})
})
public class ExamplePlugin implements Interceptor {
  private Properties properties = new Properties();
  public Object intercept(Invocation invocation) throws Throwable {
    // implement pre processing if need

    Object returnObject = invocation.proceed();

    // implement post processing if need
    return returnObject;
  }

  public void setProperties(Properties properties) {
    this.properties = properties;
  }
}

---------mybatis全局xml配置
<plugins>
    <plugin interceptor="org.format.mybatis.cache.interceptor.ExamplePlugin"></plugin>
</plugins>

自定义一个mybatis的拦截器步骤包含：

• 定义实现org.apache.ibatis.plugin.Interceptor接口的类。
• 在mybatis的全局配置xml中配置插件plugin。

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);
}


public interface ParameterHandler {

  Object getParameterObject();
  void setParameters(PreparedStatement ps) throws SQLException;
}


public interface ResultSetHandler {

  <E> List<E> handleResultSets(Statement stmt) throws SQLException;
  <E> Cursor<E> handleCursorResultSets(Statement stmt) throws SQLException;
  void handleOutputParameters(CallableStatement cs) throws SQLException;
}


public interface StatementHandler {

  Statement prepare(Connection connection, Integer transactionTimeout) throws SQLException;
  void parameterize(Statement statement) throws SQLException;
  void batch(Statement statement) throws SQLException;
  int update(Statement statement) hrows SQLException;
  <E> List<E> query(Statement statement, ResultHandler resultHandler) hrows SQLException;
  <E> Cursor<E> queryCursor(Statement statement) hrows SQLException;
  BoundSql getBoundSql();
  ParameterHandler getParameterHandler();
}

拦截点说明：

• 不同拦截点能够支持拦截的方法。
• 拦截方法的type、method、args等都取自上述方法。

Mybatis拦截器原理

拦截器的原理核心的内容在于拦截器配置解析、拦截器的职责链构造、拦截器的执行，串联上述功能后就能传统整体的流程。

拦截器解析

private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));  // 解析插件相关的参数
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }


  private void pluginElement(XNode parent) throws Exception {
    if (parent != null) {
      for (XNode child : parent.getChildren()) { // 解析生成拦截器
        String interceptor = child.getStringAttribute("interceptor");
        Properties properties = child.getChildrenAsProperties();
        Interceptor interceptorInstance = (Interceptor) resolveClass(interceptor).getDeclaredConstructor().newInstance();
        interceptorInstance.setProperties(properties);
        configuration.addInterceptor(interceptorInstance); // 保存拦截器
      }
    }
  }


public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>(); // 全局变量保存所有的拦截器

  // 织入拦截器
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  // 负责保存所有的拦截器
  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

  // 获取所有拦截器
  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }

}

拦截器解析说明：

• 负责解析XML配置文件解析所有的拦截器对象保存到拦截器链InterceptorChain 。
• InterceptorChain通过pluginAll植入拦截器。

拦截器织入

public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    // 插入拦截器
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    // 插入拦截器
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    // 插入拦截器
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }

    // 插入拦截器
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

拦截器织入说明：

• 针对ParameterHandler 、ResultSetHandler 、StatementHandler 、Executor 的拦截器的植入都是通过interceptorChain.pluginAll来实现。

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>();

  public Object pluginAll(Object target) {
    // 遍历所有的拦截器负责进行植入
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }
}

public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;
  // 通过wrap织入单个拦截器
  default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }

  default void setProperties(Properties properties) {
    // NOP
  }

}

public class Plugin implements InvocationHandler {

  public static Object wrap(Object target, Interceptor interceptor) {
    // 解析拦截器的注解获取拦截的拦截点和对应的拦截方法
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();

    // 获取被拦截对应的信息构建JDK的动态代理
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      // 针对注解指定的interface进行拦截
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }
}

拦截器织入说明：

• 拦截器织入依赖于JDK自带的Proxy.newProxyInstance动态代理来实现。
• 返回的对象是个Plugin对象的动态代理。

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    // 1、获取所有拦截的Intercepts注解
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
    if (interceptsAnnotation == null) {
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
    }
    // 2、获取注解的内容
    Signature[] sigs = interceptsAnnotation.value();
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
    // 3、遍历注解按照type + method维度进行构建
    for (Signature sig : sigs) {
      Set<Method> methods = MapUtil.computeIfAbsent(signatureMap, sig.type(), k -> new HashSet<>());
      try {
        Method method = sig.type().getMethod(sig.method(), sig.args());
        methods.add(method);
      } catch (NoSuchMethodException e) {
        throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
      }
    }
    return signatureMap;
  }


  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<>();
    while (type != null) {
      // 获取拦截点对应的Interfaces并且在拦截点注解配置的的interface用以构建动态代理
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[0]);
  }

拦截器织入说明：

• 通过解析注解@Intercepts注解@Signature来构建class + method两个维度的map对象。
• 以@Signature(type = ParameterHandler.class, method = "setParameters", args = PreparedStatement.class)为例，会构建key为ParameterHandler，value为setParameters的map

拦截器执行

public class Plugin implements InvocationHandler {

  private final Object target;
  private final Interceptor interceptor;
  private final Map<Class<?>, Set<Method>> signatureMap;

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 判断是否是被拦截的方法，如果是就通过拦截器进行处理
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        // 由拦截器interceptor负责处理
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }
}

JDK动态代理

public class Main {
    public static void main(String[] args) throws Exception {
        
        final Dog dog = new Dog();

        IDog proxy = (IDog)Proxy.newProxyInstance(Dog.class.getClassLoader(), Dog.class.getInterfaces(), new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

                return method.invoke(dog, args);
            }
        });

        proxy.eat();

        Thread.sleep(100000);
    }
}


-------------------------对应生成的动态代码

package com.sun.proxy;

import com.sunboy.IDog;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;

public final class $Proxy0 extends Proxy implements IDog {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    private static Method m0;

    public $Proxy0(InvocationHandler invocationHandler) {
        super(invocationHandler);
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("com.sunboy.IDog").getMethod("eat", new Class[0]);
            m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
            m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
            return;
        }
        catch (NoSuchMethodException noSuchMethodException) {
            throw new NoSuchMethodError(noSuchMethodException.getMessage());
        }
        catch (ClassNotFoundException classNotFoundException) {
            throw new NoClassDefFoundError(classNotFoundException.getMessage());
        }
    }

    public final boolean equals(Object object) {
        try {
            return (Boolean)this.h.invoke(this, m1, new Object[]{object});
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final String toString() {
        try {
            return (String)this.h.invoke(this, m2, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final int hashCode() {
        try {
            return (Integer)this.h.invoke(this, m0, null);
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }

    public final void eat() {
        try {
            this.h.invoke(this, m3, null);
            return;
        }
        catch (Error | RuntimeException throwable) {
            throw throwable;
        }
        catch (Throwable throwable) {
            throw new UndeclaredThrowableException(throwable);
        }
    }
}

• JDK动态代理生成的代理类如$Proxy0，通过arthas去反编译类。
• JDK动态代理通过反射获取Method对象用以后续的执行。