java - In what order are the different parts of a class initialized when a class is loaded in the JVM?

Question

Imagine a Java class which has most features that you can find in a class. For example: it inherits from another class, implements a couple of interfaces, includes some 'static final' constants, some final constants, some static variables, instance variables, a static block, an unnamed code block (just code in {}), constructors, methods etc.

When the class in question is loaded into the JVM for the first time, in what order are the various portions of the class initialized or loaded into the JVM? What does the call stack in the JVM look like for the loading? Assume that only one classloader is at work here.

This is going back to the absolute basics/internals of Java, but I havent been able to find a good article explaining the correct sequence.

Answer 1

This could be described in the section 2.17.4 of the JVMS 5.0/6

2.17.4 Initialization

Initialization of a class consists of:

• executing its static initializers (§2.11) and
• the initializers for static fields (§2.9.2) declared in the class.

Initialization of an interface consists of executing the initializers for fields declared in the interface (§2.13.3.1).

Before a class or interface is initialized, its direct superclass must be initialized, but interfaces implemented by the class need not be initialized. Similarly, the superinterfaces of an interface need not be initialized before the interface is initialized.

A class or interface type T will be initialized immediately before one of the following occurs:

• T is a class and an instance of T is created.
• T is a class and a static method of T is invoked.
• A nonconstant static field of T is used or assigned. A constant field is one that is (explicitly or implicitly) both final and static, and that is initialized with the value of a compile-time constant expression. A reference to such a field must be resolved at compile time to a copy of the compile-time constant value, so uses of such a field never cause initialization.

Invocation of certain methods in library classes (§3.12) also causes class or interface initialization. See the Java 2 platform's class library specifications (for example, class Class and package java.lang.reflect) for details.

The intent here is that a type have a set of initializers that put it in a consistent state and that this state be the first state that is observed by other classes. The static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope. This restriction is designed to detect, at compile time, most circular or otherwise malformed initializations.

Before a class or interface is initialized its superclass is initialized, if it has not previously been initialized.

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The updated version of Initialization in JVMS 8 is in Chapter 5.5

Initialization of a class or interface consists of executing its class or interface initialization method (§2.9).

A class or interface may be initialized only as a result of:

• The execution of any one of the Java Virtual Machine instructions new, getstatic, putstatic, or invokestatic that references the class or interface (§new, §getstatic, §putstatic, §invokestatic).
  All of these instructions reference a class directly or indirectly through either a field reference or a method reference.
  Upon execution of a new instruction, the referenced class or interface is initialized if it has not been initialized already.
  Upon execution of a getstatic, putstatic, or invokestatic instruction, the class or interface that declared the resolved field or method is initialized if it has not been initialized already.
• The first invocation of a java.lang.invoke.MethodHandle instance which was the result of resolution of a method handle by the Java Virtual Machine (§5.4.3.5) and which has a kind of 2 (REF_getStatic), 4 (REF_putStatic), 6 (REF_invokeStatic), or 8 (REF_newInvokeSpecial).
• Invocation of certain reflective methods in the class library (§2.12), for example, in class Class or in package java.lang.reflect.
• The initialization of one of its subclasses.
• Its designation as the initial class at Java Virtual Machine start-up (§5.2).

Prior to initialization, a class or interface must be linked, that is, verified, prepared, and optionally resolved.

Because the Java Virtual Machine is multithreaded, initialization of a class or interface requires careful synchronization, since some other thread may be trying to initialize the same class or interface at the same time.
There is also the possibility that initialization of a class or interface may be requested recursively as part of the initialization of that class or interface.

The implementation of the Java Virtual Machine is responsible for taking care of synchronization and recursive initialization by using the following procedure.
It assumes that the Class object has already been verified and prepared, and that the Class object contains state that indicates one of four situations:

• This Class object is verified and prepared but not initialized.
• This Class object is being initialized by some particular thread.
• This Class object is fully initialized and ready for use.
• This Class object is in an erroneous state, perhaps because initialization was attempted and failed.