聊聊Mybatis的binding模块
为什么我们在使用Mybatis的时候只需要写接口和xml文件就能执行sql呢？这就是Mybatis的binding模块需要做的事情了，今天我们分析一下Mybatis的binding模块，binding包下的类主要有四个MapperRegistry、MapperProxyFactory、MapperProxy和MapperMethod
映射注册类MapperRegistry
MapperRegistry是个注册类，它的knownMappers集合保存着Mapper接口和MapperProxyFactory实例的映射关系
它的addMapper()方法就是添加映射关系的方法：
public void addMapper(Class 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<>(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);
}
}
}
} 当传入的type是个接口并knownMappers中没有，就把type和对应的MapperProxyFactory实例放入knownMappers中
然后在执行sql的时候，Mybatis会调用MapperRegistry.getMapper()方法
@SuppressWarnings("unchecked")
public T getMapper(Class type, SqlSession sqlSession) {
final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
} 先从knownMappers集合中找到对应的MapperProxyFactory实例
然后调用newInstance()方法
映射代理工厂类MapperProxyFactory
MapperProxyFactory的newInstance()方法：public T newInstance(SqlSession sqlSession) {
final MapperProxy mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy mapperProxy) {
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
} 生成mapperInterface接口 的代理对象的实例，代理类是MapperProxy，它实现了InvocationHandler，利用jdk动态代理生成代理对象
看一下它重写的invoke()方法：
映射代理类MapperProxy
MapperProxy的invoke()方法：
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else {
return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}不是Object的类会被拦截，拦截调用cachedInvoker()方法，其他执行invoke()方法，
private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
try {
return MapUtil.computeIfAbsent(methodCache, method, m -> {
if (m.isDefault()) {
try {
if (privateLookupInMethod == null) {
return new DefaultMethodInvoker(getMethodHandleJava8(method));
} else {
return new DefaultMethodInvoker(getMethodHandleJava9(method));
}
} catch (IllegalAccessException | InstantiationException | InvocationTargetException
| NoSuchMethodException e) {
throw new RuntimeException(e);
}
} else {
return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
}
});
} catch (RuntimeException re) {
Throwable cause = re.getCause();
throw cause == null ? re : cause;
}
}从methodCache中获取对应的MapperMethodInvoker
如果缓存没有，如果是方法是default方法就创建DefaultMethodInvoker对象，否则创建PlainMethodInvoker对象
默认方法调用类DefaultMethodInvoker
对于DefaultMethodInvoker类通过MethodHandle完成调用private static class DefaultMethodInvoker implements MapperMethodInvoker {
private final MethodHandle methodHandle;
public DefaultMethodInvoker(MethodHandle methodHandle) {
super();
this.methodHandle = methodHandle;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return methodHandle.bindTo(proxy).invokeWithArguments(args);
}
}对于普通方法对应的类PlainMethodInvoker通过MapperMethod来完成调用，
private static class PlainMethodInvoker implements MapperMethodInvoker {
private final MapperMethod mapperMethod;
public PlainMethodInvoker(MapperMethod mapperMethod) {
super();
this.mapperMethod = mapperMethod;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
return mapperMethod.execute(sqlSession, args);
}
}总结
这篇文章主要介绍了聊聊Mybatis的binding模块，它的MapperRegistry保存着Mapper接口和MapperProxyFactory实例的映射关系，而MapperProxyFactory是一个的工厂，生成的是Mapper接口的代理类，MapperProxy实现InvocationHandler，重写invoke()方法进行拦截处理，根据方法判断是不是default类型创建不同的MethodInvoker类，然后调用MapperMethod执行sql，下篇文章我们将介绍MapperMethod类