javac之BridgeMethod及泛型擦除重写

When compiling a class or interface that extends a parameterized class or implements a parameterized interface, the compiler may need to create a synthetic method, called a bridge method, as part of the type erasure process. You normally don't need to worry about bridge methods, but you might be puzzled if one appears in a stack trace.

public class Node<T> {

    public T data;

    public Node(T data) { this.data = data; }

    public void setData(T data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

public class MyNode extends Node<Integer> {
    public MyNode(Integer data) { super(data); }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }
}

After type erasure, the Node and MyNode classes become:

public class Node {

    public Object data;

    public Node(Object data) { this.data = data; }

    public void setData(Object data) {
        System.out.println("Node.setData");
        this.data = data;
    }
}

class MyNode extends Node {

    public MyNode(Integer data) {
        super(data);
    }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }

    /*synthetic*/ public void setData(Object x0) {
        this.setData((Integer)x0);
    }
}

　　

After type erasure, the method signatures do not match. The Node method becomes setData(Object) and the MyNode method becomes setData(Integer). Therefore, the MyNodesetData method does not override the Node setData method.

To solve this problem and preserve the polymorphism of generic types after type erasure, a Java compiler generates a bridge method to ensure that subtyping works as expected. For the MyNode class, the compiler generates the following bridge method for setData:

class MyNode extends Node {

    // Bridge method generated by the compiler
    //
    public void setData(Object data) {
        setData((Integer) data);
    }

    public void setData(Integer data) {
        System.out.println("MyNode.setData");
        super.setData(data);
    }

    // ...
}

As you can see, the bridge method, which has the same method signature as the Node class's setData method after type erasure, delegates to the original setData method.

下面来看个泛型擦除重写的例子，如下：

class Cell<A> {
	A value;
}

class Test {
	public void t() {
		Cell<Integer> cell = null;
		Object x1 = cell.value;
		Integer x2 = cell.value;
		int x3 = cell.value;
		long x4 = cell.value;
	}
}

最后必定为如下的格式：

class Test {
    Test() { }

    public void t() {
        Cell<Integer> cell = null;
        Object x1 = ((Cell)cell).value;
        Integer x2 = (Integer)((Cell)cell).value;
        int x3 = ((Integer)((Cell)cell).value).intValue();
        long x4 = (long)((Integer)((Cell)cell).value).intValue();
    }
}

　　

　　

参考：

（1）http://www.baeldung.com/java-type-erasure