Java中的符号引用 [英] Symbolic references in Java

问题描述

在这些日子里，我一直在玩Java反射和 .class 格式。我目前正在学习 ldc 指令。

In these days I have been playing with Java reflection and .class format. I'm currently studying ldc instruction.

在JVM规范中我发现术语我不明白：符号引用，我有以下问题。

In JVM Specification I found term I don't understand: symbolic reference, and I have the following questions.

1. 这是什么意思？

1. What does it mean?

它在哪里使用？

推荐答案

如果引用它会很有帮助给你带来麻烦的文件的确切部分。既然你还没有，我将从 ldc ：

It would be helpful if you would quote the exact piece of the documentation that's giving you trouble. Since you haven't, I'm going to take a guess at what you might have quoted, from the doc for ldc:

否则，如果是运行时常量池entry是一个符号引用
到类（第5.1节），然后命名类被解析（第5.4.3.1节）和
引用Class对象表示该类，值是
推入操作数堆栈。

Otherwise, if the run-time constant pool entry is a symbolic reference to a class (§5.1), then the named class is resolved (§5.4.3.1) and a reference to the Class object representing that class, value, is pushed onto the operand stack.

否则，运行时常量池条目必须是对方法类型或方法句柄的符号
引用（§ 5.1）。 ...

Otherwise, the run-time constant pool entry must be a symbolic reference to a method type or a method handle (§5.1). ...

此引号包含指向JVM规范（5.1）的另一部分的链接，该部分描述了运行时常量池：

This quote has a link to another section of the JVM spec (5.1), which describes the run-time constant pool:

运行时数据结构，用于传统编程语言实现的
符号表的许多用途

a run-time data structure that serves many of the purposes of the symbol table of a conventional programming language implementation

这意味着运行时常量池包含有关符号形式的类的各个部分的信息：作为文本值。

What this means is that the run-time constant pool contains information about the pieces of a class in symbolic form: as text values.

因此，当 ldc 给出一个类的符号引用时，它的索引为 CONSTANT_Class_info 常量池中的结构。如果你看一下这个结构的定义，你会看到它包含对类名的引用，也是在常量池中保存的。

So, when ldc is given a "symbolic reference" to a class, it's given the index of a CONSTANT_Class_info structure within the constant pool. If you look at the definition of this structure, you'll see that it contains a reference to the name of the class, also held within the constant pool.

TL; DR：符号引用是可用于检索实际对象的字符串。

TL;DR: "symbolic references" are strings that can be used to retrieve the actual object.

一个例子：

if (obj.getClass() == String.class) {
    // do something
}

成为以下字节码：

aload_1
invokevirtual   #21; //Method java/lang/Object.getClass:()Ljava/lang/Class;
ldc     #25; //class java/lang/String
if_acmpne       20

在这种情况下， ldc operation是指以符号形式存储的类。当JVM执行此操作码时，它将使用符号引用来标识当前类加载器中的实际类，并返回对类实例的引用。

In this case, the ldc operation refers to a class that is stored symbolically. When the JVM executes this opcode, it will use the symbolic reference to identify the actual class within the current classloader, and return a reference to the class instance.

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