使用java8

一、类加载运行全过程

当我们用java命令运行某个类的main函数启动程序时，首先需要通过类加载器把主类加载到JVM。

package jvmdemo;

public class Math {
    public static final int initData = 666;
    public static User user = new User();

    public int compute() {  //一个方法对应一块栈帧内存区域
        int a = 1;
        int b = 2;
        int c = (a + b) * 10;
        return c;
    }

    public static void main(String[] args) {
        Math math = new Math();
        math.compute();
    }
}

1.1通过Java命令执行代码的大体流程

流程图地址

1.2 loadClass的类加载过程有如下几步

加载 >> 验证 >> 准备 >> 解析 >> 初始化 >> 使用 >> 卸载
● 加载：在硬盘上查找并通过IO读入字节码文件，使用到类时才会加载，例如调用类的main()方法，new对象等等，在加载阶段会在内存中生成一个代表这个类的java.lang.Class对象，作为方法区这个类的各种数据的访问入口
● 验证：校验字节码文件的正确性
● 准备：给类的静态变量分配内存，并赋予默认值
● 解析：将符号引用替换为直接引用，该阶段会把一些静态方法(符号引用，比如main()方法)替换为指向数据所存内存的指针或句柄等(直接引用)，这是所谓的静态链接过程(类加载期间完成)，动态链接是在程序运行期间完成的将符号引用替换为直接引用
● 初始化：对类的静态变量初始化为指定的值，执行静态代码块

类被加载到方法区中后主要包含 运行时常量池、类型信息、字段信息、方法信息、类加载器的引用、对应class实例的引用等信息。
类加载器的引用：这个类到类加载器实例的引用
对应class实例的引用：类加载器在加载类信息放到方法区中后，会创建一个对应的Class 类型的对象实例放到堆(Heap)中, 作为开发人员访问方法区中类定义的入口和切入点。
注意：主类在运行过程中如果使用到其它类，会逐步加载这些类。
jar包或war包里的类不是一次性全部加载的，是使用到时才加载。

package jvmdemo;

public class TestDynamicLoad {

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

    public static void main(String[] args) {
        new A();
        System.out.println("*************load test************");
        B b = null;  //B不会加载，除非这里执行 new B()
    }
}

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

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

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

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

运行结果：

二、类加载器和双亲委派机制

2.1 Java里几种类加载器

● 引导类加载器(BootstrapLoader)：负责加载支撑JVM运行的位于JRE的lib目录下的核心类库，比如rt.jar、charsets.jar等
● 扩展类加载器(ExtClassloader)：负责加载支撑JVM运行的位于JRE的lib目录下的ext扩展目录中的JAR类包
● 应用程序类加载器(AppClassLoader)：负责加载ClassPath路径下的类包，主要就是加载你自己写的那些类
● 自定义加载器：负责加载用户自定义路径下的类包

import sun.misc.Launcher;
import java.net.URL;

public class TestJDKClassLoader {
    public static void main(String[] args) {
        System.out.println(String.class.getClassLoader());
        System.out.println(com.sun.crypto.provider.DESKeyFactory.class.getClassLoader().getClass().getName());
        System.out.println(TestJDKClassLoader.class.getClassLoader().getClass().getName());

        System.out.println();
        ClassLoader appClassLoader = ClassLoader.getSystemClassLoader();
        ClassLoader extClassloader = appClassLoader.getParent();
        ClassLoader bootstrapLoader = extClassloader.getParent();
        System.out.println("the bootstrapLoader : " + bootstrapLoader);
        System.out.println("the extClassloader : " + extClassloader);
        System.out.println("the appClassLoader : " + appClassLoader);

        System.out.println();
        System.out.println("bootstrapLoader加载以下文件：");
        URL[] urls = Launcher.getBootstrapClassPath().getURLs();
        for (int i = 0; i < urls.length; i++) {
            System.out.println(urls[i]);
        }

        System.out.println();
        System.out.println("extClassloader加载以下文件：");
        System.out.println(System.getProperty("java.ext.dirs"));

        System.out.println();
        System.out.println("appClassLoader加载以下文件：");
        System.out.println(System.getProperty("java.class.path"));

    }
}

运行结果

null
sun.misc.Launcher$ExtClassLoader
sun.misc.Launcher$AppClassLoader

the bootstrapLoader : null
the extClassloader : sun.misc.Launcher$ExtClassLoader@511d50c0
the appClassLoader : sun.misc.Launcher$AppClassLoader@7f31245a

bootstrapLoader加载以下文件：
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/sunrsasign.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/classes

extClassloader加载以下文件：
/Users/wanghuaikun/Library/Java/Extensions:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java

appClassLoader加载以下文件：
/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_77.jdk/Contents/Home/lib/tools.jar:/Users/online/whk1demo/target/classes:/Users/wanghuaikun/.m2/repository/org/springframework/boot/spring-boot-starter-web/3.1.1/spring-boot-starter-web-3.1.1.jar:/Users/wanghuaikun/.m2/repository/org/springframework/boot/spring-boot-starter/3.1.1/spring-boot-starter-3.1.1.jar:/Users/wanghuaikun/.m2/repository/org/springframework/boot/spring-boot/3.1.1/spring-boot-3.1.1.jar:/Users/wanghuaikun/.m2/repository/org/springframework/boot/spring-boot-autoconfigure/3.1.1/spring-boot-autoconfigure-3.1.1.jar:/Users/wanghuaikun/.m2/repository/org/springframework/boot/spring-boot-starter-logging/3.1.1/spring-boot-starter-logging-3.1.1.jar:/Users/wanghuaikun/.m2/repository/ch/qos/logback/logback-classic/1.4.8/logback-classic-1.4.8.jar:/Users/wanghuaikun/.m2/repository/ch/qos/logback/logback-core/1.4.8/logback-core-1.4.8.jar:/Users/wanghuaikun/.m2/repository/org/apache/logging/log4j/log4j-to-slf4j/2.20.0/log4j-to-slf4j-2.20.0.jar:/Users/wanghuaikun/.m2/repository/org/apache/logging/log4j/log4j-api/2.20.0/log4j-api-2.20.0.jar:/Users/wanghuaikun/.m2/repository/org/slf4j/jul-to-slf4j/2.0.7/jul-to-slf4j-2.0.7.jar:/Users/wanghuaikun/.m2/repository/jakarta