类初始化缘起

//: typeinfo/ClassInitialization.java
import java.util.*;

class Initable {
	static final int staticFinal = 47;
	static final int staticFinal2 = ClassInitialization.rand.nextInt(1000);
	static {
		System.out.println("Initializing Initable");
	}
}

class Initable2 {
	static int staticNonFinal = 147;
	static {
		System.out.println("Initializing Initable2");
	}
}

class Initable3 {
	static int staticNonFinal = 74;
	static {
		System.out.println("Initializing Initable3");
	}
}

public class ClassInitialization {
	public static Random rand = new Random(47);

	public static void main(String[] args) throws Exception {
		Class initable = Initable.class;
		System.out.println("After creating Initable ref");
		// Does not trigger initialization:
		System.out.println(Initable.staticFinal);
		// Does trigger initialization:
		System.out.println(Initable.staticFinal2);
		// Does trigger initialization:
		System.out.println(Initable2.staticNonFinal);
		Class initable3 = Class.forName("Initable3");
		System.out.println("After creating Initable3 ref");
		System.out.println(Initable3.staticNonFinal);
	}
}

 

可以看到，获得一个对class literal的引用不会导致类初始化，一般而言，引用static final的“编译时”变量也不会引起类初始化

 

Effectively, initialization is "as lazy as possible." From the creation of the initable reference, you can see that just using the .class syntax to get a reference to the class doesn’t cause initialization. However, Class.forName( ) initializes the class immediately in order to produce the Class reference, as you can see in the creation of initable3.
If a static final value is a "compile-time constant," such as Initable.staticFinal, that value can be read without causing the Initable class to be initialized. Making a field static and final, however, does not guarantee this behavior: accessing Initable.staticFinal2 forces class initialization because it cannot be a compile-time constant.
If a static field is not final, accessing it always requires linking (to allocate storage for the field) and initialization (to initialize that storage) before it can be read, as you can see in the access to Initable2.staticNonFinal.