Symbolic references in Java

I've been playing java reflection and reflection these days Class format I'm learning the LDC command

In the JVM specification, I find that I don't understand the term: symbolic reference. I have the following problems

What does that mean? > Where does it work? > In which case does the LDC instruction load symbol references? > Is there any code corresponding to this action in Java?

Solution

It will be helpful if you quote some articles that cause you trouble Since you don't, I'll guess what you may quote from the LDC document:

This reference has a link to another part of the JVM specification (5.1), which describes the runtime constant pool:

This means that the runtime constant pool contains some information about the symbolic form of the class: as a text value

Therefore, when LDC gives a "symbolic reference" to a class, it is given constant in the constant pool_ Class_ Index of info structure If you look at the definition of this structure, you will see that it contains a reference to the class name and is also saved in the constant pool

TL; Dr: a symbolic reference is a string that can be used to retrieve an actual object

An example:

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

Become the following bytecode:

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

In this case, the LDC operation refers to a class stored in symbolic form When the JVM executes this opcode, it will use a symbolic reference to identify the actual class in the current class loader and return a reference to the class instance