JVM学习笔记7-类的卸载

类的卸载

指类从内存中注销掉,注销完成后将不能再创建类的实例对象
       当MySample类加载,连接和初始化后,它的生命周期就开始了.
       若代表MySample类的Class对象不再被引用,即不可触及时,Class对象就会结束生命周期,MySample类在方法区内的数据也会被卸载,从而结束MySample类的生命周期

---

一个类何时结束生命周期,取决于代表他的Class对象何时结束生命周期

---

• 由JVM自带的类加载器所加载的类,在JVM生命周期类,始终不会被卸载
• JVM自带的类加载器包括根类加载器,扩展类类加载器,系统类加载器
• JVM本身会始终引用自带的类加载器,而这些类加载器则会始终引用他们所加载的类的Class对象—即这些Class对象始终是可触及的
• 由用户自定义的类加载器所加载的类是可以被卸载的

public class MyTest16 extends ClassLoader {
    private String classLoaderName;

    private final String fileExtension = ".class";

    private String path;

    public String getPath() {
        return this.path;
    }

    public void setPath(String path) {
        this.path = path;
    }

    public MyTest16(String classLoaderName) {
        //将系统类加载器当做该类加载器的父加载器
        super();
        this.classLoaderName = classLoaderName;
    }

    public MyTest16(ClassLoader parent, String classLoaderName) {
        //显示指定该类加载器的父加载器
        super(parent);
        this.classLoaderName = classLoaderName;
    }

    private byte[] loadClassData(String className) {
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = new ByteArrayOutputStream();

        className = className.replaceAll("\\.", "/");

        try {
            String filePath = this.path + className + fileExtension;
            is = new FileInputStream(new File(filePath));
            int ch = 0;
            while ((ch = is.read()) != -1) {
                baos.write(ch);
            }
            data = baos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }
        return data;
    }

    @Override
    protected Class<?> findClass(String className) throws ClassNotFoundException {
        System.out.println("findClass invoked : "+ className);
        byte[] data = this.loadClassData(className);
        return this.defineClass(className, data, 0, data.length);
    }

    @Override
    public String toString() {
        return "MyTest16{" +
                "classLoaderName='" + classLoaderName + '\'' +
                ", fileExtension='" + fileExtension + '\'' +
                '}';
    }

    public static void main(String[] args) throws Exception {
        MyTest16 load1 = new MyTest16("load1");
//        load1.setPath("/Users/lutingfeng/Documents/idea/jvm_learn/out/production/jvm_learn/main/jvm/classloader");
        load1.setPath("/Users/lutingfeng/Desktop/");

        Class<?> clazz = load1.loadClass("main.jvm.classloader.MyTest1");
        System.out.println("clazz.hashCode="+clazz.hashCode());
        Object obj = clazz.newInstance();
        System.out.println(obj);
        System.out.println("classLoad="+obj.getClass().getClassLoader());

        load1 = null;
        clazz = null;
        obj = null;

        System.gc();

        load1 = new MyTest16("load1");
//        load1.setPath("/Users/lutingfeng/Documents/idea/jvm_learn/out/production/jvm_learn/main/jvm/classloader");
        load1.setPath("/Users/lutingfeng/Desktop/");

        clazz = load1.loadClass("main.jvm.classloader.MyTest1");
        System.out.println("clazz.hashCode="+clazz.hashCode());
        obj = clazz.newInstance();
        System.out.println(obj);
        System.out.println("classLoad="+obj.getClass().getClassLoader());

    }
}

在JVM参数中加上-XX:+TraceClassUnloading并删除classPath下的MyTest1.class文件后执行结果:

findClass invoked : main.jvm.classloader.MyTest1
clazz.hashCode=2125039532
main.jvm.classloader.MyTest1@12a3a380
classLoad=MyTest16{classLoaderName=‘load1’, fileExtension=’.class’}
[Unloading class main.jvm.classloader.MyTest1 0x00000007c0060828]
findClass invoked : main.jvm.classloader.MyTest1
clazz.hashCode=1846274136
main.jvm.classloader.MyTest1@61bbe9ba
classLoad=MyTest16{classLoaderName=‘load1’, fileExtension=’.class’}

可见自定义ClassLoader—MyTest16所加载的类MyTest1被卸载

JVM学习笔记8-常用指令,助记符,JVM参数