【深入理解Java虚拟机】类加载机制_java 虚拟机 机制

class SuperClass{
    static{
        System.out.println("SuperClass init!");
    }
    public static int value = 123;
}
 
class SubClass extends SuperClass{
    static{
        System.out.println("SubClass init!");//子类中引用父类的静态字段，不会导致类初始化
    }
}
 
public class Test {
    public static void main(String[] args) {
        System.out.println(SubClass.value);
    }
}

class ConstClass{
    static{
        System.out.println("ConstClass init!");
    }
    public static final String HELLOWORLD = "hello world";
}
 
public class Test {
    public static void main(String[] args) {
        System.out.println(ConstClass.HELLOWORLD);
    }
}

public class ClassLoaderTest {  
    public static void main(String[] args) throws Exception {  
        ClassLoader myLoader = new ClassLoader() {  
            @Override  
            public Class<?> loadClass(String name)  
                    throws ClassNotFoundException {  
                try {  
                    String fileName = name.substring(name.lastIndexOf(".") + 1)  
                            + ".class";  
                    InputStream is = getClass().getResourceAsStream(fileName);  
                    if (is == null) {  
                        return super.loadClass(name);  
                    }  
                    byte[] b = new byte[is.available()];  
                    is.read(b);  
                    return defineClass(name, b, 0, b.length);  
                } catch (IOException e) {  
                    throw new ClassNotFoundException(name);  
                }  
            }  
        };  
 
        Class c = myLoader.loadClass("org.bupt.xiaoye.blog.ClassLoaderTest");  
        Object obj = c.newInstance();  
        System.out.println(obj.getClass());  
        System.out.println(ClassLoaderTest.class);  
        System.out.println(obj instanceof ClassLoaderTest);  
 
    }  
}

protected Class<?> loadClass(String name, boolean resolve)  
        throws ClassNotFoundException {  
    synchronized (getClassLoadingLock(name)) {  
        // 首先检查类是否已经被加载过  
        Class c = findLoadedClass(name);  
        if (c == null) {  
            long t0 = System.nanoTime();  
            try {  
                if (parent != null) {  
                    // 调用父类加载器加载  
                    c = parent.loadClass(name, false);  
                } else {  
                    c = findBootstrapClassOrNull(name);  
                }  
            } catch (ClassNotFoundException e) {  
                // ClassNotFoundException thrown if class not found  
                // from the non-null parent class loader  
            }  
 
            if (c == null) {  
                // If still not found, then invoke findClass in order  
                // to find the class.  
                //父类加载器无法完成加载，调用本身的加载器加载
                long t1 = System.nanoTime();  
                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;  
    }  
}