jvm源码浅谈

浅谈jvm

java虚拟机加载源码分析

  /**
   * 首先c++创建java虚拟机和引导类加载(BootStrapApplication),然后引导类加载器加载Launcher,通过Launcher创建其他类加载器。
   */
  1.getLauncher()单例的Lanuncher
   public static Launcher getLauncher() {
        return launcher;
    }
  2.初始化Lanucher中的扩展类加载器(ExtClassLoader)和应用类加载器(AppClassLoader)
    public Launcher() {
    var1 = Launcher.ExtClassLoader.getExtClassLoader();
    this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
    }
  3.getAppClassLoader(var1)方法默认从classpath加载资源,var1是扩展类加载器，扩展类加载器此处指定是确定应用类加载器默认生成的时候继承了ClassLoader类的parent属性，parent=ExtClassLoader,同理ExtClassLoader.parent=BootStrapClassLoader=null
   public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException {
            final String var1 = System.getProperty("java.class.path");
            final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1);
            return (ClassLoader)AccessController.doPrivileged(new PrivilegedAction<Launcher.AppClassLoader>() {
                public Launcher.AppClassLoader run() {
                    URL[] var1x = var1 == null ? new URL[0] : Launcher.pathToURLs(var2);
                    return new Launcher.AppClassLoader(var1x, var0);
                }
            });
        }

        AppClassLoader(URL[] var1, ClassLoader var2) {
            super(var1, var2, Launcher.factory);
            this.ucp.initLookupCache(this);
        }

双亲委派机制源码解析

//Layncher类加载loadClass(name,?) 安全校验代码省略，看核心代码
   public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
                return super.loadClass(var1, var2);
            }
        }
//ClassLoader
protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            //首先第一次加载查看是否已加载，加载完毕直接返回，否则交父类加载器加载。(判断是否加载过)
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                    	//父类加载器不为null 直接交有弗雷加载器重新执行LoadClass
                        c = parent.loadClass(name, false);
                    } else {
                    	//ext期待类加载器无法加载，直接交由引导类加载器加载
                        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.
                    long t1 = System.nanoTime();
                    //加载器开始加载类，抛出异常ClassNotFoundException
                    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;
        }
    }

双亲委派首先判断类是否被加载，应用类加载器没有被加载才会交给parent(ext),ext未加载交给parent(bootStrap)

面试

package java.lang;
public class String {
    public static void main(String[] args) {
        System.out.println("hello world");
    }
}
//运行结果
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

1.自定义类加载器加载Class类 MyClassLoader 自定义类加载器继承ClassLoader类，实现findClass方法，重写具体的业务逻辑，代码如下：

public class MyClassLoader extends ClassLoader {
	//获取类的加载路径
    private String classPath;
    public MyClassLoader(String classPath) {
        this.classPath = classPath;
    }
	//把需要加载的class文件Io流读取
    private byte[] loadByte(String name) throws Exception {
      	//加载路径替换
        name = name.replaceAll("\\.", "/");
        //加载类的Class文件,转化为字节数据
        FileInputStream fis = new FileInputStream(classPath + "/" + name
                + ".class");
        int len = fis.available();
        byte[] data = new byte[len];
        fis.read(data);
        fis.close();
        return data;
    }
    //类加载器在在使用自定义的findClass加载器加载文件
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        try {
            byte[] data = loadByte(name);
            //defineClass将一个字节数组转为Class对象，这个字节数组是class文件读取后最终的字节
            return defineClass(name, data, 0, data.length);
        } catch (Exception e) {
            e.printStackTrace();
            throw new ClassNotFoundException();
        }
    }
}

如何打破双亲委派机制？ tomcat容器加载多个war就是采用了此思想。

//首选需要重写ClassLoader类的findClass方法，之后写业务逻辑
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();
                try {
                //打破双亲委派机制，如果加载的name是以此name开头的，采用自定义的findClass直接加载到jvm虚拟机，否则继续执行双亲委派机制。
                    if (name.startsWith("com.hello.jvm")) {
                        c = findClass(name);
                    } else {
                        c = this.getParent().loadClass(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.
                    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;
        }
    }

双亲委派机制： 主要是为了防止外部人员修改JDK源码级别的东西，防止入侵，还有就是避免了类的重复加载。完结！！！