带你了解Mybatis中getMapper()的来龙去脉

getMapper用法在熟悉不过了，指明一个DAO接口的class，调用他的方法，即可从数据库中返回 ，是不是感觉很神奇？
其实都是通过JDK的动态代理来实现，getMapper返回的即一个代理对象，通常在写动态代理时，代理对象处理完成后还有调用被代理对象的对应方法，而像Mybatis这种面向接口的思想，没有被代理的对象，所以，Mybatis通过自己一系列操作后直接返回。

源码分析

一、加载MappedStatement

首先Mybatis要加载mapper下的所有insert、select、update、delete标签，并且封装成一个个MappedStatement对象，保存在Configuration下的Map<String, MappedStatement>中，具体实现在 XMLStatementBuilder#parseStatementNode() 和 MapperBuilderAssistant#addMappedStatement() 的方法中(addMappedStatement参数不多，也就20个),其中 this.configuration.addMappedStatement()就是保存在Configuration配置类中

image 这里的Map键为namespace+标签的id，他在MapperBuilderAssistant#applyCurrentNamespace() 方法中被拼接，value即一个MappedStatement对象
Configuration的addMappedStatement方法 拼接id 还有之后在XMLMapperBuilder#bindMapperForNamespace() 保存mapper的namespace，存放在Configuration的mapperRegistry字段中的knownMappers下。knownMapper是一个map，key为namespace，value为MapperProxyFactory对象。
image

二、getMapper()

getMapper()从DefaultSqlSession下的getMapper()中开始，中间经过Configuration的getMapper()，最终调用到MapperRegistry下的getMapper();其中knownMappers中的值在上述addMapper()中被添加。

public class MapperRegistry {
    private final Configuration config;
    private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap();
    public MapperRegistry(Configuration config) {
        this.config = config;
    }
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        //从已知的集合中返回mapper代理工厂，
        MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
        //为空则抛出异常
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        } else {
            try {
                //实例化代理对象
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception var5) {
                throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
            }
        }
 ........
}

动态代理生成工厂，通过newInstance实例化代理对象

public class MapperProxyFactory<T> {
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap();

    public MapperProxyFactory(Class<T> mapperInterface) {
        this.mapperInterface = mapperInterface;
    }

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

    public Map<Method, MapperMethod> getMethodCache() {
        return this.methodCache;
    }
    
    protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
    }

    public T newInstance(SqlSession sqlSession) {
        //JDK的动态代理实现，代理对象为MapperProxy。
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }
}

所以在调用DAO中方法时，都会转入到MapperProxy的invoke方法下。
执行过程中判断了是不是java8中的default方法或者是不是Object下的方法。如果都不是，才去查询，

public class MapperProxy<T> implements InvocationHandler, Serializable {
    private static final long serialVersionUID = -6424540398559729838L;
    private final SqlSession sqlSession;
    private final Class<T> mapperInterface;
    private final Map<Method, MapperMethod> methodCache;

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

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
        //如果是Object中的方法，则直接执行代理类的对象的对应方法
            if (Object.class.equals(method.getDeclaringClass())) {
                return method.invoke(this, args);
            }
        //如果是默认方法，也就是java8中的default方法
            if (this.isDefaultMethod(method)) {
                return this.invokeDefaultMethod(proxy, method, args);
            }
        } catch (Throwable var5) {
            throw ExceptionUtil.unwrapThrowable(var5);
        }
        //从缓存中获取MapperMethod
        MapperMethod mapperMethod = this.cachedMapperMethod(method);
        return mapperMethod.execute(this.sqlSession, args);
    }
    

这里先要从methodCache获取对应DAO方法的MapperMethod。

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

紧接着是生成MapperMethod.SqlCommand和MapperMethod.MethodSignature，两个自己的内部类，
在SqlCommand中，更具id从Configuration中获取MappedStatement，此时就是上述第一步中添加的MappedStatement，完成代码层和XML中的关系映射，并把MappedStatement的id和type提取出来，如果没有获取到，抛出异常，也就是为什么DAO中有select，xml配置中没有id为select抛出异常的原因。

public class MapperMethod {
   private final MapperMethod.SqlCommand command;
   private final MapperMethod.MethodSignature method;
   public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
       this.command = new MapperMethod.SqlCommand(config, mapperInterface, method);
       this.method = new MapperMethod.MethodSignature(config, mapperInterface, method);
   }
//.........
 public static class SqlCommand {
       private final String name;
       private final SqlCommandType type;

       public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
           String methodName = method.getName();
           Class<?> declaringClass = method.getDeclaringClass();
           //从Configuration中获取MappedStatement
           MappedStatement ms = this.resolveMappedStatement(mapperInterface, methodName, declaringClass, configuration);
           if (ms == null) {
               if (method.getAnnotation(Flush.class) == null) {
                   throw new BindingException("Invalid bound statement (not found): " + mapperInterface.getName() + "." + methodName);
               }

               this.name = null;
               this.type = SqlCommandType.FLUSH;
           } else {
               this.name = ms.getId();
               this.type = ms.getSqlCommandType();
               if (this.type == SqlCommandType.UNKNOWN) {
                   throw new BindingException("Unknown execution method for: " + this.name);
               }
           }

       }

更具不同的标签类型，执行不同的方法

image