1. main方法执行流程

2. 类加载过程

/**
 * 测试静态方法和构造方法哪个先加载
 */
public class Math {

    static{
        System.out.println("load Math");
    }

    public static void main(String[] args) {
        new A();
        System.out.println("load test");
    }

}

class A{
    static {
        System.out.println("load A");
    }

    public A(){
        System.out.println("initial A");
    }
}

3.类加载器和类加载过程(TODO)

4. 为什么要设计双亲委派机制？

package java.lang;

/**
 * 错误: 在类 java.lang.String 中找不到 main 方法, 请将 main 方法定义为:
 *    public static void main(String[] args)
 * 否则 JavaFX 应用程序类必须扩展javafx.application.Application
 */
public class String {

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

5.自定义加载器(TODO)

import java.io.FileInputStream;
import java.io.IOException;
import java.lang.reflect.Method;

public class MyClassLoader extends ClassLoader{

    private String path;//类路径

    public MyClassLoader(String path) {
        this.path = path;
    }

    @Override
    protected Class<?> findClass(String name) {
        //加载类文件二进制数据
        byte[] data = new byte[0];
        try {
            data = loadBytes(name);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return defineClass(name,data,0, data.length);
    }

    private byte[] loadBytes(String name) throws IOException {
        name = name.replaceAll("\\.","/");
        FileInputStream fis = new FileInputStream(path + "/" + name + ".class");
        int len = fis.available();

        byte[] data = new byte[len];

        fis.read(data);
        fis.close();
        return  data;
    }

    public static void main(String[] args) throws Exception {
        MyClassLoader myClassLoader = new MyClassLoader("D:/test");
        Class<?> aClass = myClassLoader.loadClass("com.test.User");
        Object obj = aClass.newInstance();
        Method method = aClass.getDeclaredMethod("sout", null);
        method.invoke(obj,null);
        System.out.println(aClass.getClassLoader().getClass().getName());
    }
}

6.打破双亲委派机制

import java.io.FileInputStream;
import java.io.IOException;
import java.lang.reflect.Method;

public class MyClassLoader extends ClassLoader{

    private String path;//类路径

    public MyClassLoader(String path) {
        this.path = path;
    }

    @Override
    protected Class<?> findClass(String name) {
        //加载类文件二进制数据
        byte[] data = new byte[0];
        try {
            data = loadBytes(name);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return defineClass(name,data,0, data.length);
    }

    private byte[] loadBytes(String name) throws IOException {
        name = name.replaceAll("\\.","/");
        FileInputStream fis = new FileInputStream(path + "/" + name + ".class");
        int len = fis.available();

        byte[] data = new byte[len];

        fis.read(data);
        fis.close();
        return  data;
    }

    /**
     * 打破双亲委派机制
     * @param name
     * @param resolve
     * @return
     * @throws ClassNotFoundException
     */
    protected Class<?> loadClass(String name, boolean resolve)
            throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();

                    //判断如果不是指定的路径走双亲委派机制，如果是走自定义类加载
                    if(!name.startsWith("com.test")){
                        c = this.getParent().loadClass(name);
                    }else{
                        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;
        }
    }

    public static void main(String[] args) throws Exception {
        MyClassLoader myClassLoader = new MyClassLoader("D:/test");
        Class<?> aClass = myClassLoader.loadClass("com.test.User");
        Object obj = aClass.newInstance();
        Method method = aClass.getDeclaredMethod("sout", null);
        method.invoke(obj,null);
        System.out.println(aClass.getClassLoader().getClass().getName());
    }
}