为什么注释字符串值没有实现？ [英] Why are annotation string values not interned?

问题描述

以下代码段打印出4个不同的哈希码，尽管重用了字符串常量和字面值。为什么字符串值没有在注释元素上实现？

The following snippet prints 4 distinct hash codes, despite reusing a string constant and literal. Why are string values not interned on annotation elements?

public class Foo {
    @Retention(RetentionPolicy.RUNTIME)
    @interface Bar {
        String CONSTANT = "foo";

        String value() default CONSTANT;
    }

    public static void main(String[] args) throws Exception {
        System.out.println(System.identityHashCode(Bar.CONSTANT));
        System.out.println(System.identityHashCode(Foo.class.getMethod("test1").getAnnotation(Bar.class).value()));
        System.out.println(System.identityHashCode(Foo.class.getMethod("test2").getAnnotation(Bar.class).value()));
        System.out.println(System.identityHashCode(Foo.class.getMethod("test3").getAnnotation(Bar.class).value()));
    }

    @Bar
    public void test1() {}

    @Bar("foo")
    public void test2() {}

    @Bar(Bar.CONSTANT)
    public void test3() {}
}

推荐答案

字符串文字是实体的，但注释需要解析，它们存储在字节数组中。如果您查看类 java.lang.reflect.Method ，您可以看到：

String literal are interned but annotations are subject to parse and they are stored in byte arrays. If you look at the class java.lang.reflect.Method you can see this:

private byte[]              annotations;
private byte[]              parameterAnnotations;
private byte[]              annotationDefault;  

另请参阅方法 public Object getDefaultValue（）。流程一直持续到这里
AnnotationParser.parseConst 并输入

Take also a look at the method public Object getDefaultValue() of the same class to see how the AnnotationParser is called. The flow continues till here AnnotationParser.parseConst and enter in

case 's':
  return constPool.getUTF8At(constIndex);

方法 ConstantPool.getUTF8At 是一个委托本土方法。你可以在这里看到代码本机实现getUFT8At 。解析的常量永远不会被实现，并且永远不会从StringTable（其中字符串被实现）中检索。

The method ConstantPool.getUTF8At is a delegate to a native method. You can see the code here native implementation getUFT8At. The parsed constant is never interned and is never retrieved from the StringTable (where string are interned).

我认为它可以是实现的选择。已经创建了Interning以在String literal之间进行更快速的比较，因此仅用于在方法实现中可用的实体文字。

I think it could be a choice of implementation. Interning has been created to make more fast comparison between String literal and so is used only for interning literal usable in method implementation.

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