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.
这是什么意思？
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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