How Java linker works?

Question

I want to know how Java linker works. Specifically, in which order it combines classes, interfaces, packages, methods and etc into jvm-executable format. I have found some information here, but there is not so much information about linking order.

Answer 1

There is no such thing as a Java "linker". There is, however, the concept of a classloader which - given an array of java byte codes from "somewhere" - can create an internal representation of a Class which can then be used with new etc.

In this scenario interfaces are just special classes. Methods and fields are available when the class has been loaded.

Answer 2

First of all: methods are always part of a class. Interfaces are basically just special classes, and packages are just a part of the fully qualified name of a class with some impact on visibility and the physical organization of class files.

So the question comes down to: how does a JVM link class files? The JVM spec you linked to says:

The Java programming language allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that the semantics of the language are respected, that a class or interface is completely verified and prepared before it is initialized, and that errors detected during linkage are thrown at a point in the program where some action is taken by the program that might require linkage to the class or interface involved in the error.

For example, an implementation may choose to resolve each symbolic reference in a class or interface individually, only when it is used (lazy or late resolution), or to resolve them all at once, for example, while the class is being verified (static resolution). This means that the resolution process may continue, in some implementations, after a class or interface has been initialized.

Thus, the question can only be answered for a specific JVM implementation.

Furthermore, it should never make a difference in the behaviour of Java programs, except possibly for the exact point where linking errors result in runtime Error instances being thrown.

Answer 3

Java doesn't do linking the way C does. The principle unit is the class definition. A lot of the matching of a class reference to its definition happens at runtime. So you could compile a class against one version of a library, but provide another version at runtime. If the relevant signatures match, everything will be ok. There's some in-lining of constants at compile time, but that's about it.

Answer 4

As noted previously Java compiler doesn't have a linker. However, JVM has a linking phase, which performed after class loading. JVM spec defines it at best:

Linking a class or interface involves verifying and preparing that class or interface, its direct superclass, its direct superinterfaces, and its element type (if it is an array type), if necessary. Resolution of symbolic references in the class or interface is an optional part of linking.

This specification allows an implementation flexibility as to when linking activities (and, because of recursion, loading) take place, provided that all of the following properties are maintained:

• A class or interface is completely loaded before it is linked.

• A class or interface is completely verified and prepared before it is initialized.

• Errors detected during linkage are thrown at a point in the program where some action is taken by the program that might, directly or indirectly, require linkage to the class or interface involved in the error.

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html#jvms-5.4