How do you disable lazy class loading/initialization in Sun's JVM?

问题

By default, Sun's JVM both lazily loads classes and lazily initializes (i.e. calls their <clinit> methods) them. Consider the following class, ClinitBomb, which throws an Exception during a static{} block.

public class ClinitBomb {
    static {
        explode();
    }   
    private static void explode() {
        throw new RuntimeException("boom!");
    }       
}

Now, consider how to trigger the bomb:

public class Main {
    public static void main(String[] args) {
        System.out.println("A");
        try {
            Class.forName("ClinitBomb");
        } catch (Exception e) {
            e.printStackTrace(System.out);
        }
        System.out.println("B");
        ClinitBomb o2 = new ClinitBomb();
        System.out.println("C");
    }
}

We're guaranteed the explosion happens before point B, since forName's documentation says so; the question is whether it happens before point A (when Main is loaded.) In Sun's JVM, even though main() contains a static reference to ClinitBomb, it happens after A.

I want a way to tell the JVM to load and initialize ClinitBomb as soon as it initializes Main (so the bomb explodes before point A.) In general, I want a way to say, "whenever loading/initializing class X, also do so for any classes Y it references."

Is there a way to do that?

回答1:

There is no way to do this. The JLS says, in §12.4.1 When Initialization Occurs (emphasis mine):

Initialization of a class consists of executing its static initializers and the initializers for static fields declared in the class. [...]

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

T is a class and an instance of T is created.

T is a class and a static method declared by T is invoked.

A static field declared by T is assigned.

A static field declared by T is used and the field is not a constant variable (§4.12.4).

T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

Invocation of certain reflective methods in class Class and in package java.lang.reflect also causes class or interface initialization. A class or interface will not be initialized under any other circumstance.

A Java implementation which initialized classes as soon as they were loaded would violate the JLS.

Although what you could do would be to use the JVM instrumentation API to write a ClassFileTransformer which added a static block to every class which explicitly initialized its referenced classes (via Class.forName, probably). As soon as one class gets initialized, all the classes reachable from it will be initialized. That might give you the result you're after. It's quite a lot of work, though!

回答2:

Class.forName("...", true /*initialize*/, getClassLoader());

You were halfways there.

来源：https://stackoverflow.com/questions/8497660/how-do-you-disable-lazy-class-loading-initialization-in-suns-jvm

标签

java

class

classloader

static-initializer