2. 类加载器子系统

/**
 *示例代码
 */
public class HelloLoader {
    public static void main(String[] args) {
        System.out.println("谢谢ClassLoader加载我...");
        System.out.println("你的大恩大德，我下辈子再报！");
    }
}

public class HelloApp {
    private static int a = 1; //prepare: a = 0 ---> initial : a = 1

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

public class ClassInitTest {
    private static int num = 1;

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

}

public class ClassInitTest {

    private static int num = 1;

    static {
        num = 2;
        number = 20;
        // System.out.println(number); // 报错，非法的前向引用 Illegal forward reference
    }

    private static int number = 10; // prepare: number = 0 --> initial: 20 --> 10

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

 0 iconst_1
 1 putstatic #3 <org/depsea/jvm/chapter02/java/ClassInitTest.num : I>
 4 iconst_2
 5 putstatic #3 <org/depsea/jvm/chapter02/java/ClassInitTest.num : I>
 8 bipush 20
10 putstatic #5 <org/depsea/jvm/chapter02/java/ClassInitTest.number : I>
13 bipush 10
15 putstatic #5 <org/depsea/jvm/chapter02/java/ClassInitTest.number : I>
18 return

public class ClinitTest {
    private int a = 10;

    public static void main(String[] args) {
        int b = 2;
    }
}

public class ClinitTest {

    private int a = 10;

    private static int c = 20;

    public static void main(String[] args) {
        int b = 2;
    }

}

public class ClinitTest {

    private int a = 10;
    private int num = 0;

    private static int c = 20;

    public ClinitTest() {
    }

    public ClinitTest(int num) {
        this.num = num;
    }

    public static void main(String[] args) {
        int b = 2;
    }
}

0 aload_0
1 invokespecial #1 <java/lang/Object.<init>>
4 aload_0
5 bipush 10
7 putfield #2 <org/depsea/jvm/chapter02/ClinitTest.a>
10 aload_0
11 iconst_0
12 putfield #3 <org/depsea/jvm/chapter02/ClinitTest.num>
15 return

0 aload_0
1 invokespecial #1 <java/lang/Object.<init>>
4 aload_0
5 bipush 10
7 putfield #2 <org/depsea/jvm/chapter02/ClinitTest.a>
10 aload_0
11 iconst_0
12 putfield #3 <org/depsea/jvm/chapter02/ClinitTest.num>
15 aload_0
16 iload_1
17 putfield #3 <org/depsea/jvm/chapter02/ClinitTest.num>
20 return

public class ClinitExtendsTest {

    static class Father {
        public static int A = 1;

        static {
            A = 2;
        }
    }

    static class Son extends Father {
        public static int B = A;
    }

    public static void main(String[] args) {
        // 加载Father类，其次加载Son类
        System.out.println(Son.B);
    }
}

0 getstatic #2 <com/atguigu/ClinitExtendsTest$Son.A : I>
3 putstatic #3 <com/atguigu/ClinitExtendsTest$Son.B : I>
6 return

0 aload_0
1 invokespecial #1 <com/atguigu/ClinitExtendsTest$Father.<init> : ()V>
4 return

public class DeadThreadTest {

    public static void main(String[] args) {
        Runnable r = () -> {
            System.out.println(Thread.currentThread().getName() + "开始");
            DeadThread dead = new DeadThread();
            System.out.println(Thread.currentThread().getName() + "结束");
        };

        Thread t1 = new Thread(r, "Thread 1 ");
        Thread t2 = new Thread(r, "Thread 2 ");

        t1.start();
        t2.start();
    }


}

class DeadThread {
    static {
        if (true) {
            System.out.println(Thread.currentThread().getName() + "初始化当前类");
        }
        // while (true) {} // 我的代码中加上这段后报错。
    }
}

public class ClassLoaderTest {

    public static void main(String[] args) {
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader); 1

        ClassLoader extClassLoader = systemClassLoader.getParent();
        System.out.println(extClassLoader); 2

        ClassLoader bootstrapClassLoader = extClassLoader.getParent();
        System.out.println(bootstrapClassLoader); 3
    }
}

public class ClassLoaderTest1 {

    public static void main(String[] args) {
        System.out.println("******启动类加载器************");

        URL[] urls = sun.misc.Launcher.getBootstrapClassPath().getURLs();
        for (URL url : urls) {
            System.out.println(url.toExternalForm()); //(1)
        }

        // 从上面的路径中随意找一个类，查看其类加载器
        ClassLoader classLoader = Provider.class.getClassLoader();
        System.out.println(classLoader); //(2)

        System.out.println("************扩展类加载器************");
        String extDirs = System.getProperty("java.ext.dirs");

        // Windows下是：";" ，Mac或者Linux下是：":"
        for (String path : extDirs.split(":")) {
            System.out.println(path); //(3)
        }

        // 从上面的路径中随意找一个类，查看其类加载器
        ClassLoader classLoader1 = CurveDB.class.getClassLoader();
        System.out.println(classLoader1); //(4)
    }
}

public class CustomClassLoader extends ClassLoader {

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {

        try {
            byte[] classBody = this.getClassFromCusomPath(name);
            if (classBody == null || classBody.length == 0) {
                throw new FileNotFoundException();
            } else {
                return this.defineClass(name, classBody, 0, classBody.length);
            }

        } catch (FileNotFoundException e) {
            throw new ClassNotFoundException(name, e);
        }
    }

    private byte[] getClassFromCusomPath(String name) {
        // 从自定义路径中加载类
        // 如果执行路径的字节码文件进行了加密，则需要在此方法进行解密操作。
        return null;
    }

    public static void main(String[] args) {
        CustomClassLoader classLoader = new CustomClassLoader();

        try {
            Class<?> clazz = Class.forName("One", true, classLoader);
            Object obj = clazz.newInstance();
            System.out.println(obj.getClass().getClassLoader());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

public class ClassLoaderTest2 {

    public static void main(String[] args) {
        try {
            // 1
            ClassLoader classLoader = Class.forName("java.lang.String").getClassLoader();
            System.out.println(classLoader);

            // 2
            ClassLoader classLoader1 = Thread.currentThread().getContextClassLoader();
            System.out.println(classLoader1);

            // 3
            ClassLoader classLoader2 = ClassLoader.getSystemClassLoader().getParent();
            System.out.println(classLoader2);

        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

package java.lang;

public class String {

    static {
        System.out.println("我是自定义的String类的静态代码块");
    }
}

public class StringTest {

    public static void main(String[] args) {
        String str = new String();
        System.out.println("hello, atguigu.com");
    }
}

public class ShkStart {

    public static void main(String[] args) {
        System.out.println("Hello!");
    }
}

Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.lang
	at java.lang.ClassLoader.preDefineClass(ClassLoader.java:662)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:761)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
	at org.depsea.jvm.chapter02.java1.ShkStartTest.main(ShkStartTest.java:25)