“.class” way of obtaining Class - does it initialize the class?

问题

Question is about Java. There are three ways of obtaining classes of a type (note that code is just a collection of snippets for demonstration):

Class c1 = Class.forName("com.mypkg.MyClass"); //assumes initialize = true
// - OR - 
Class c1 = Class.forName("com.mypkg.MyClass", true/false,
                         this.getClass().getClassLoader());

Class c2 = com.mypkg.MyClass.class;
// - OR - 
import com.mypkg.MyClass;
Class c2 = MyClass.class;

MyClass mc = new MyClass();
Class c3 = mc.getClass();

My question is about initialization. For method 1, I can control whether class is initialized or not using the boolean argument. For method 3, since an object is created, obviously the class is initialized (right?).

But what about method 2? Does calling .class initialize the class if it has not already been initialized?
And how can one programmatically check whether a class is already initialized or not?

Summary answer:

Please see accepted solution and its comments for full answer. This is just a summary.

Calling .class does not initialize the class if it has not already been initialized. You can check whether a class is getting initialized by using a static block in the class definition that prints a message.

Reason for original question:

According to javadoc of Class.forName, "A call to forName("X") causes the class named X to be initialized.". Also there are two Class.forName methods including one that accepts a boolean parameter called initialize. According to javadoc, "The class is initialized only if the initialize parameter is true and if it has not been initialized earlier."

回答1:

How about testing it yourself, to see what your VM does?

Just use this class with all three methods (in independent runs, as it will only be initialized once at most!)

class Example {
    static {
        System.out.println("Class was initialized!");
    }

    public static int bananas = 0;
}

for details when a class is supposed to be initialized, see http://docs.oracle.com/javase/specs/#12.4.1

Roughly speaking, a class is supposed to be initialized when either

the first instance is created
a static method is invoked
a non-final static field is used
some more complex corner cases

So essentially, when you first use anything of the actual class, not just reference it.

But e.g. Example.bananas = 1; should also trigger class initialization, without having an instance.

回答2:

On the bytecode level, the reference to the class is loaded by the one of the ldc instructions. The VM specification does not mention that the classes are initialized, so the safe thing to say is probably that the VM does not guarantee that such a reference initializes the class, but do mind that the VM specification does not require a VM to do initialization as lazily as possible either.

The class may legally be initialized at any time between it being first referenced and initialization being strictly required.

来源：https://stackoverflow.com/questions/13739726/class-way-of-obtaining-class-does-it-initialize-the-class

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