Mybatis 源码之Plugin类解析

public class Plugin implements InvocationHandler {

private Object target; //目标对象
private Interceptor interceptor;//拦截器对象
private Map<Class<?>, Set<Method>> signatureMap;//目标对象方法签名

private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
this.target = target;
this.interceptor = interceptor;
this.signatureMap = signatureMap;
}

public static Object wrap(Object target, Interceptor interceptor) {
//从拦截器的注解中获取拦截的类名和方法信息
Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
Class<?> type = target.getClass();
//解析被拦截对象的所有接口
Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
if (interfaces.length > 0) {
//生成代理对象，Plugin对象为该代理对象的InvocationHandler
return Proxy.newProxyInstance(
type.getClassLoader(),
interfaces,
new Plugin(target, interceptor, signatureMap));
}
return target;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
Set<Method> methods = signatureMap.get(method.getDeclaringClass());
if (methods != null && methods.contains(method)) {
//调用代理类所属拦截器的intercept方法
return interceptor.intercept(new Invocation(target, method, args));
}
return method.invoke(target, args);
} catch (Exception e) {
throw ExceptionUtil.unwrapThrowable(e);
}
}

private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
//获得Interceptor注解，@Signature中的type(要拦截的类),method(拦截类的方法)和args(拦截器用于这些类中)
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());
}
//获得注解type,method 和args生成一个signature数组
Signature[] sigs = interceptsAnnotation.value();
Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();
for (Signature sig : sigs) {
Set<Method> methods = signatureMap.get(sig.type());
if (methods == null) {
methods = new HashSet<Method>();
signatureMap.put(sig.type(), methods);
}
try {
//获得类的方法
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;
}
//获得所有接口
private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
Set<Class<?>> interfaces = new HashSet<Class<?>>();
while (type != null) {
//获得接口
for (Class<?> c : type.getInterfaces()) {
if (signatureMap.containsKey(c)) {
interfaces.add(c);
}
}
//获得父类
type = type.getSuperclass();
}
//返回一个接口的数组
return interfaces.toArray(new Class<?>[interfaces.size()]);
}

}