jvm虚拟机类加载机制详解

public class NotInitialization {
    public static void main(String[] args) {
        System.out.println(SubClass.value);  // 只会输出“SuperClass init”，而不会输出“subclass init”
        SuperClass[] superClasses = new SuperClass[10];  // 不会输出“SuperClass init”
    }
}

class SuperClass {
    static {
        System.out.println("SuperClass init");
    }
    public static int value = 123;
}

class SubClass extends SuperClass {
    static {
        System.out.println("SubClass init!");
    }
}

public class NotInitialization {
    public static void main(String[] args) {
        System.out.println(ConstClass.HELLOWORLD);  // 不会输出“ConstClass init!”，因为常量直接存储到常量池中。
    }
}
class ConstClass {
    static {
        System.out.println("ConstClass init!");
    }

    public static final String HELLOWORLD = "hello world";
}

class Test {
    static {
        i = 0;
    }
    static int i = 1;
}

public static void main(String[] args) {
    System.out.println(Sub.B);
}

static class Parent {
    public static int A = 1;
    static {
        A = 2;
    }
}

static class Sub extends Parent {
    public static int B = A;
}

private final ClassLoader parent; 
protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // 首先，检查请求的类是否已经被加载过
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {//父加载器不为空，调用父加载器loadClass()方法处理
                        c = parent.loadClass(name, false);
                    } else {//父加载器为空，使用启动类加载器 BootstrapClassLoader 加载
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                   //抛出异常说明父类加载器无法完成加载请求
                }
                
                if (c == null) {
                    long t1 = System.nanoTime();
                    //自己尝试加载
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }