【JVM】（2）java类加载机制

java 复制代码

 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 {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        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();
                    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;
        }
    }

java 复制代码

package com.roy.classLoader;

public class LoaderDemo {
    public static String a ="aaa";
    public static void main(String[] args) throws ClassNotFoundException {
        // 父子关系 AppClassLoader <- ExtClassLoader <- BootStrap Classloader
        ClassLoader cl1 = LoaderDemo.class.getClassLoader();
        System.out.println("cl1 > " + cl1);
        System.out.println("parent of cl1 > " + cl1.getParent());
        // BootStrap Classloader由C++开发，是JVM虚拟机的一部分，本身不是JAVA类。
        System.out.println("grant parent of cl1 > " + cl1.getParent().getParent());
        // String,Int等基础类由BootStrap Classloader加载。
        ClassLoader cl2 = String.class.getClassLoader();
        System.out.println("cl2 > " + cl2);
        System.out.println(cl1.loadClass("java.util.List").getClass().getClassLoader());

        // java指令可以通过增加-verbose:class -verbose:gc 参数在启动时打印出类加载情况
        // BootStrap Classloader，加载java基础类。这个属性不能在java指令中指定，推断不是由java语言处理。。
        System.out.println("BootStrap ClassLoader加载目录：" + System.getProperty("sun.boot.class.path"));
        // Extention Classloader 加载JAVA_HOME/ext下的jar包。 可通过-D java.ext.dirs另行指定目录
        System.out.println("Extention ClassLoader加载目录：" + System.getProperty("java.ext.dirs"));
        // AppClassLoader 加载CLASSPATH，应用下的Jar包。可通过-D java.class.path另行指定目录
        System.out.println("AppClassLoader加载目录：" + System.getProperty("java.class.path"));
    }
}