MyBatis Mapper 接口如何通过JDK动态代理来包装SqlSession 源码分析

我们以往使用ibatis或者mybatis 都是以这种方式调用XML当中定义的CRUD标签来执行SQL 比如这样

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
  PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.mybatis.example.BlogMapper">
  <select id="selectBlog" resultType="Blog">
    select * from Blog where id = #{id}
  </select>
</mapper>

SqlSession session = sqlSessionFactory.openSession();
try {
  Blog blog = (Blog) session.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);
} finally {
  session.close();
}

这种方式有很明显的缺点就是通过字符串去调用标签定义的SQL，第一容易出错，第二是当XML当中的id修改过以后你不知道在程序当中有多少个地方使用了这个ID,需要手工查找并一一修改。在Mybatis这个版本中做了一些改进，支持这种方式调用。

定义一个接口 方法名，参数需要与XML定义保持一致。

org.mybatis.example.BlogMapper.selectBlog

public interface BlogMapper {
  Blog selectBlog(int id);
}

然后这么调用，这样以来当我们修改了XML的ID以后，只需要修改接口中的方法就可以了，编译器会在其他使用该接口的地方报错，很容易进行修改。当然好处还不只这些，还可以通过与Spring进行无缝集成，动态注入 等等。后面会一一讲到。

SqlSession session = sqlSessionFactory.openSession();
try {
  BlogMapper mapper = session.getMapper(BlogMapper.class);
  Blog blog = mapper.selectBlog(101);
} finally {
  session.close();
}

本文的重点不是去讲解如何使用MyBatis(关于如何使用Mybatis可以参考官方API http://mybatis.github.io/mybatis-3/zh/getting-started.html),而是讲解MyBatis是如何通过接口对SqlSession进行动态封装的。

在上面的例子当中呢，BlogMapper是一个接口 它并没有实现类，为什么接口可以直接使用呢? 

那是因为MyBbatis使用了JDK动态代理机制动态生成了代理类，那么代理类又是如何多SqlSession进行封装的呢?

带着这些疑问，让我们通过分析源代码的方式来解释这些问题。

Mybatis关于包装Mapper的代码都在org.apache.ibatis.binding 这个包下面。

其中有4个类。

上面的4个类封装了Mapper接口动态生成代理类的全部细节

MapperRegistry 类是注册Mapper接口与获取代理类实例的工具类

package org.apache.ibatis.binding;

import org.apache.ibatis.builder.annotation.MapperAnnotationBuilder;
import org.apache.ibatis.io.ResolverUtil;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.SqlSession;

import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

//这个类通过名字就可以看出 是用来注册Mapper接口与获取生成代理类实例的工具类 
public class MapperRegistry {

  //全局配置文件对象
  private Configuration config;

  //一个HashMap Key是mapper的类型对象, Value是MapperProxyFactory对象
  //这个MapperProxyFactory是创建Mapper代理对象的工厂 我们一会在分析
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();

  public MapperRegistry(Configuration config) {
    this.config = config;
  }

  //获取生成的代理对象
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //通过Mapper的接口类型 去Map当中查找 如果为空就抛异常
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null)
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    try {
      //否则创建一个当前接口的代理对象 并且传入sqlSession
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
  
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }

  //注册Mapper接口
  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<T>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }

    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  //通过包名扫描下面所有接口
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }
  
}

package org.apache.ibatis.binding;

import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.ibatis.session.SqlSession;

//这个类负责创建具体Mapper接口代理对象的工厂类
public class MapperProxyFactory<T> {

  //具体Mapper接口的Class对象
  private final Class<T> mapperInterface;
  //该接口下面方法的缓存 key是方法对象 value是对接口中方法对象的封装
  private Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();

  //构造参数没啥好说的
  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    //创建了一个代理类并返回
    //关于Proxy的API 可以查看java官方的API
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  //在这里传入sqlSession 创建一个Mapper接口的代理类
  public T newInstance(SqlSession sqlSession) {
    //在这里创建了MapperProxy对象 这个类实现了JDK的动态代理接口 InvocationHandler
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    //调用上面的方法 返回一个接口的代理类
    return newInstance(mapperProxy);
  }

}

package org.apache.ibatis.binding;

import java.io.Serializable;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.util.Map;

import org.apache.ibatis.session.SqlSession;

//实现了JDK动态代理的接口 InvocationHandler
//在invoke方法中实现了代理方法调用的细节
public class MapperProxy<T> implements InvocationHandler, Serializable {

  private static final long serialVersionUID = -6424540398559729838L;
  //SqlSession
  private final SqlSession sqlSession;
  //接口的类型对象
  private final Class<T> mapperInterface;

  //接口中方法的缓存 有MapperProxyFactory传递过来的。
  private final Map<Method, MapperMethod> methodCache;

  //构造参数
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }

  //接口代理对象所有的方法调用 都会调用该方法
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    //判断是不是基础方法 比如toString() hashCode()等，这些方法直接调用不需要处理
    if (Object.class.equals(method.getDeclaringClass())) {
      return method.invoke(this, args);
    }
    //这里进行缓存
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //调用mapperMethod.execute 核心的地方就在这个方法里，这个方法对才是真正对SqlSession进行的包装调用
    return mapperMethod.execute(sqlSession, args);
  }

  //缓存处理
  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }

}

package org.apache.ibatis.binding;

import org.apache.ibatis.annotations.MapKey;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.mapping.MappedStatement;
import org.apache.ibatis.mapping.SqlCommandType;
import org.apache.ibatis.reflection.MetaObject;
import org.apache.ibatis.session.Configuration;
import org.apache.ibatis.session.ResultHandler;
import org.apache.ibatis.session.RowBounds;
import org.apache.ibatis.session.SqlSession;

import java.lang.reflect.Array;
import java.lang.reflect.Method;
import java.util.*;

//这个类是整个代理机制的核心类，对Sqlsession当中的操作进行了封装
public class MapperMethod {

  //一个内部封 封装了SQL标签的类型 insert update delete select
  private final SqlCommand command;
  //一个内部类 封装了方法的参数信息 返回类型信息等
  private final MethodSignature method;

  //构造参数
  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, method);
  }

  //这个方法是对SqlSession的包装调用
  public Object execute(SqlSession sqlSession, Object[] args) {
    //定义返回结果
    Object result;
    //如果是INSERT操作
    if (SqlCommandType.INSERT == command.getType()) {
      //处理参数
      Object param = method.convertArgsToSqlCommandParam(args);
      //调用sqlSession的insert方法
      result = rowCountResult(sqlSession.insert(command.getName(), param));

      //如果是UPDATE操作 同上
    } else if (SqlCommandType.UPDATE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.update(command.getName(), param));

      //如果是DELETE操作 同上
    } else if (SqlCommandType.DELETE == command.getType()) {
      Object param = method.convertArgsToSqlCommandParam(args);
      result = rowCountResult(sqlSession.delete(command.getName(), param));

      //如果是SELECT操作 那么情况会多一些 但是也都和sqlSession的查询方法一一对应
    } else if (SqlCommandType.SELECT == command.getType()) {
      //如果返回void 并且参数有resultHandler
      //则调用 void select(String statement, Object parameter, ResultHandler handler);方法
      if (method.returnsVoid() && method.hasResultHandler()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      //如果返回多行结果这调用 <E> List<E> selectList(String statement, Object parameter);
      //executeForMany这个方法调用的
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      //如果返回类型是MAP 则调用executeForMap方法
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else {
        //否则就是查询单个对象
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
      }
    } else {
      //如果全都不匹配 说明mapper中定义的方法不对
      throw new BindingException("Unknown execution method for: " + command.getName());
    }
    //如果返回值为空 并且方法返回值类型是基础类型 并且不是VOID 则抛出异常
    if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
      throw new BindingException("Mapper method '" + command.getName() 
          + " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
    }
    return result;
  }

  private Object rowCountResult(int rowCount) {
    final Object result;
    if (method.returnsVoid()) {
      result = null;
    } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
      result = rowCount;
    } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
      result = (long) rowCount;
    } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
      result = (rowCount > 0);
    } else {
      throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
    }
    return result;
  }

  private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
    MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
    if (void.class.equals(ms.getResultMaps().get(0).getType())) {
      throw new BindingException("method " + command.getName() 
          + " needs either a @ResultMap annotation, a @ResultType annotation," 
          + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
    }
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
    } else {
      sqlSession.select(command.getName(), param, method.extractResultHandler(args));
    }
  }

  //返回多行结果 调用sqlSession.selectList方法
  private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    //如果参数含有rowBounds则调用分页的查询
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<E>selectList(command.getName(), param, rowBounds);
    } else {
      //没有分页则调用普通查询
      result = sqlSession.<E>selectList(command.getName(), param);
    }
    // issue #510 Collections & arrays support
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }

  private <E> Object convertToDeclaredCollection(Configuration config, List<E> list) {
    Object collection = config.getObjectFactory().create(method.getReturnType());
    MetaObject metaObject = config.newMetaObject(collection);
    metaObject.addAll(list);
    return collection;
  }

  @SuppressWarnings("unchecked")
  private <E> E[] convertToArray(List<E> list) {
    E[] array = (E[]) Array.newInstance(method.getReturnType().getComponentType(), list.size());
    array = list.toArray(array);
    return array;
  }

  private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
    Map<K, V> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    if (method.hasRowBounds()) {
      RowBounds rowBounds = method.extractRowBounds(args);
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey(), rowBounds);
    } else {
      result = sqlSession.<K, V>selectMap(command.getName(), param, method.getMapKey());
    }
    return result;
  }

  public static class ParamMap<V> extends HashMap<String, V> {

    private static final long serialVersionUID = -2212268410512043556L;

    @Override
    public V get(Object key) {
      if (!super.containsKey(key)) {
        throw new BindingException("Parameter '" + key + "' not found. Available parameters are " + keySet());
      }
      return super.get(key);
    }

  }

  //一个内部类 封装了具体执行的动作
  public static class SqlCommand {

    //xml标签的id
    private final String name;
    //insert update delete select的具体类型
    private final SqlCommandType type;


    public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) throws BindingException {
      //拿到全名 比如 org.mybatis.example.BlogMapper.selectBlog
      String statementName = mapperInterface.getName() + "." + method.getName();
      MappedStatement ms = null;
      //获取MappedStatement对象 这个对象封装了XML当中一个标签的所有信息 比如下面
      //<select id="selectBlog" resultType="Blog">
      //select * from Blog where id = #{id}
      //</select>
      if (configuration.hasStatement(statementName)) {
        ms = configuration.getMappedStatement(statementName);
      } else if (!mapperInterface.equals(method.getDeclaringClass().getName())) { // 这里是一个BUG
        String parentStatementName = method.getDeclaringClass().getName() + "." + method.getName();
        if (configuration.hasStatement(parentStatementName)) {
          ms = configuration.getMappedStatement(parentStatementName);
        }
      }
      //为空抛出异常
      if (ms == null) {
        throw new BindingException("Invalid bound statement (not found): " + statementName);
      }
      name = ms.getId();
      type = ms.getSqlCommandType();
      //判断SQL标签类型 未知就抛异常
      if (type == SqlCommandType.UNKNOWN) {
        throw new BindingException("Unknown execution method for: " + name);
      }
    }

    public String getName() {
      return name;
    }

    public SqlCommandType getType() {
      return type;
    }
  }

  //内部类 封装了接口当中方法的 参数类型 返回值类型 等信息
  public static class MethodSignature {

    //是否返回多调结果
    private final boolean returnsMany;
    //返回值是否是MAP
    private final boolean returnsMap;
    //返回值是否是VOID
    private final boolean returnsVoid;
    //返回值类型
    private final Class<?> returnType;
    //mapKey
    private final String mapKey;
    //resultHandler类型参数的位置
    private final Integer resultHandlerIndex;
    //rowBound类型参数的位置
    private final Integer rowBoundsIndex;
    //用来存放参数信息
    private final SortedMap<Integer, String> params;
    //是否存在命名参数
    private final boolean hasNamedParameters;

    //在这里对上面的属性进行初始化 就不一一详细说明了 具体实现细节可以看下面的代码。
    public MethodSignature(Configuration configuration, Method method) throws BindingException {
      this.returnType = method.getReturnType();
      this.returnsVoid = void.class.equals(this.returnType);
      this.returnsMany = (configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray());
      this.mapKey = getMapKey(method);
      this.returnsMap = (this.mapKey != null);
      this.hasNamedParameters = hasNamedParams(method);
      this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
      this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
      this.params = Collections.unmodifiableSortedMap(getParams(method, this.hasNamedParameters));
    }

    public Object convertArgsToSqlCommandParam(Object[] args) {
      final int paramCount = params.size();
      if (args == null || paramCount == 0) {
        return null;
      } else if (!hasNamedParameters && paramCount == 1) {
        return args[params.keySet().iterator().next()];
      } else {
        final Map<String, Object> param = new ParamMap<Object>();
        int i = 0;
        for (Map.Entry<Integer, String> entry : params.entrySet()) {
          param.put(entry.getValue(), args[entry.getKey()]);
          // issue #71, add param names as param1, param2...but ensure backward compatibility
          final String genericParamName = "param" + String.valueOf(i + 1);
          if (!param.containsKey(genericParamName)) {
            param.put(genericParamName, args[entry.getKey()]);
          }
          i++;
        }
        return param;
      }
    }

    public boolean hasRowBounds() {
      return (rowBoundsIndex != null);
    }

    public RowBounds extractRowBounds(Object[] args) {
      return (hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null);
    }

    public boolean hasResultHandler() {
      return (resultHandlerIndex != null);
    }

    public ResultHandler extractResultHandler(Object[] args) {
      return (hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null);
    }

    public String getMapKey() {
      return mapKey;
    }

    public Class<?> getReturnType() {
      return returnType;
    }

    public boolean returnsMany() {
      return returnsMany;
    }

    public boolean returnsMap() {
      return returnsMap;
    }

    public boolean returnsVoid() {
      return returnsVoid;
    }

    private Integer getUniqueParamIndex(Method method, Class<?> paramType) {
      Integer index = null;
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) {
        if (paramType.isAssignableFrom(argTypes[i])) {
          if (index == null) {
            index = i;
          } else {
            throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
          }
        }
      }
      return index;
    }

    private String getMapKey(Method method) {
      String mapKey = null;
      if (Map.class.isAssignableFrom(method.getReturnType())) {
        final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class);
        if (mapKeyAnnotation != null) {
          mapKey = mapKeyAnnotation.value();
        }
      }
      return mapKey;
    }

    private SortedMap<Integer, String> getParams(Method method, boolean hasNamedParameters) {
      final SortedMap<Integer, String> params = new TreeMap<Integer, String>();
      final Class<?>[] argTypes = method.getParameterTypes();
      for (int i = 0; i < argTypes.length; i++) {
        if (!RowBounds.class.isAssignableFrom(argTypes[i]) && !ResultHandler.class.isAssignableFrom(argTypes[i])) {
          String paramName = String.valueOf(params.size());
          if (hasNamedParameters) {
            paramName = getParamNameFromAnnotation(method, i, paramName);
          }
          params.put(i, paramName);
        }
      }
      return params;
    }

    private String getParamNameFromAnnotation(Method method, int i, String paramName) {
      final Object[] paramAnnos = method.getParameterAnnotations()[i];
      for (Object paramAnno : paramAnnos) {
        if (paramAnno instanceof Param) {
          paramName = ((Param) paramAnno).value();
        }
      }
      return paramName;
    }

    private boolean hasNamedParams(Method method) {
      boolean hasNamedParams = false;
      final Object[][] paramAnnos = method.getParameterAnnotations();
      for (Object[] paramAnno : paramAnnos) {
        for (Object aParamAnno : paramAnno) {
          if (aParamAnno instanceof Param) {
            hasNamedParams = true;
            break;
          }
        }
      }
      return hasNamedParams;
    }

  }

}

通过上面的分析就很容易弄清楚Mybatis是如何利用JDK动态代理的机制生成代理类来对各种Mapper接口进行封装的了。