详解Java的桥接方法

public class Parent {
  Number get() {
    return 1;
  }
}

public class Child extends Parent {

  @Override
  Integer get() {
    return 1;
  }
}

javac Child.java
javap -v -c Child.class

public class Child extends Parent
......省略部分结果......
 java.lang.Integer get();
  descriptor: ()Ljava/lang/Integer;
  flags:
  Code:
   stack=1, locals=1, args_size=1
     0: iconst_1
     1: invokestatic #2         // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
     4: areturn
   LineNumberTable:
    line 5: 0

 java.lang.Number get();
  descriptor: ()Ljava/lang/Number;
  flags: ACC_BRIDGE, ACC_SYNTHETIC
  Code:
   stack=1, locals=1, args_size=1
     0: aload_0
     1: invokevirtual #3         // Method get:()Ljava/lang/Integer;
     4: areturn
   LineNumberTable:
    line 1: 0

public class Parent<T> {

  void set(T t) {
  }
}

public class Child extends Parent<String> {

  @Override
  void set(String str) {
  }
}

public class Child extends Parent<java.lang.String>
......省略部分结果......
 void set(java.lang.String);
  descriptor: (Ljava/lang/String;)V
  flags:
  Code:
   stack=0, locals=2, args_size=2
     0: return
   LineNumberTable:
    line 5: 0

 void set(java.lang.Object);
  descriptor: (Ljava/lang/Object;)V
  flags: ACC_BRIDGE, ACC_SYNTHETIC
  Code:
   stack=2, locals=2, args_size=2
     0: aload_0
     1: aload_1
     2: checkcast   #2         // class java/lang/String
     5: invokevirtual #3         // Method set:(Ljava/lang/String;)V
     8: return
   LineNumberTable:
    line 1: 0

public class Parent<Object> {

  void set(Object t) {
  }
}

method = BridgeMethodResolver.findBridgedMethod(method);

public static Method findBridgedMethod(Method bridgeMethod) {
  // 如果不是桥连方法，就直接返回原方法。
	if (!bridgeMethod.isBridge()) {
		return bridgeMethod;
	}
  // 先从本地缓存读取，缓存中有则直接返回。
	Method bridgedMethod = cache.get(bridgeMethod);
	if (bridgedMethod == null) {
		List<Method> candidateMethods = new ArrayList<>();
    // 以方法名称和入参个数相等为筛选条件。
		MethodFilter filter = candidateMethod ->
				isBridgedCandidateFor(candidateMethod, bridgeMethod);
    // 递归该类及其所有父类上的所有方法，符合筛选条件就添加进来。
		ReflectionUtils.doWithMethods(bridgeMethod.getDeclaringClass()
      , candidateMethods::add, filter);
		if (!candidateMethods.isEmpty()) {
      // 如果符合筛选条件的方法个数为1，则直接采用；
      // 否则，调用searchCandidates方法再次筛选。
			bridgedMethod = candidateMethods.size() == 1 ?
					candidateMethods.get(0) :
					searchCandidates(candidateMethods, bridgeMethod);
		}
    // 如果找不到实际方法，则返回原来的桥连方法。
		if (bridgedMethod == null) {
			// A bridge method was passed in but we couldn't find the bridged method.
			// Let's proceed with the passed-in method and hope for the best...
			bridgedMethod = bridgeMethod;
		}
    // 把查找的结果放入内存缓存。
		cache.put(bridgeMethod, bridgedMethod);
	}
	return bridgedMethod;
}

private static Method searchCandidates(List<Method> candidateMethods, Method bridgeMethod) {
	if (candidateMethods.isEmpty()) {
		return null;
	}
	Method previousMethod = null;
	boolean sameSig = true;
  // 遍历候选方法的列表
	for (Method candidateMethod : candidateMethods) {
    // 对比桥接方法的泛型类型参数和候选方法是否匹配，如果匹配则直接返回该候选方法。
		if (isBridgeMethodFor(bridgeMethod, candidateMethod, bridgeMethod.getDeclaringClass())) {
			return candidateMethod;
		}
		else if (previousMethod != null) {
      // 如果不匹配，则判断所有候选方法的参数列表是否相等。
			sameSig = sameSig && Arrays.equals(candidateMethod.getGenericParameterTypes()
        , previousMethod.getGenericParameterTypes());
		}
		previousMethod = candidateMethod;
	}
  // 如果所有候选方法的参数列表全相等，则返回第一个候选方法。
	return (sameSig ? candidateMethods.get(0) : null);
}