JAVA ClassLoader加載機制簡單分析

一、什麼是ClassLoader：

        咱們稱其爲類加載器，咱們的java類編譯以後成爲.class的文件，ClassLoader將.class文件加載進入內存而後執行相關的代碼。

二、Java中的ClassLoader：

Java爲咱們提供了三個ClassLoader：AppClassLoader、ExtClassLoader、BootstrapClassLoader。下面咱們來看他們之間的關係。

此圖是我在網上找的最下面的CustomClassLoader是說咱們自定義的ClassLoader這裏不去管它。下面是本人關於ClassLoader的試驗代碼：

/**
 * 分別打印BootstrapClassLoader ExtClassLoader AppClassLoader的加載路徑
 * Created by zcg on 2017/7/27.
 */
public class ClassLoaderTest {
    public static void main(String[] args){
        //獲取main的類加載器
        ClassLoader classLoader = ClassLoaderTest.class.getClassLoader();
        System.out.println("main classLoader"+classLoader.toString());

        URL[] murls = ((URLClassLoader)classLoader).getURLs();
        printUrl(murls);

        System.out.println();

        //經過getParent方法獲取 parent Classloader
        ClassLoader parentLoader = classLoader.getParent();
        System.out.println("parent classLoader"+parentLoader.toString());

        URL[] murlsExt = ((URLClassLoader)parentLoader).getURLs();
        printUrl(murlsExt);

        /**用反射獲得其實也能夠直接這樣:
         *  Launcher launcher = new Launcher();
            URLClassPath urlClassPath = launcher.getBootstrapClassPath();
            System.out.println();
            printUrl(urlClassPath.getURLs());
         和下面代碼等價
        打印BootstarpClassLoader加載路徑 (由於是C++實現的)*/
        try {
            Class launcherClass = Class.forName("sun.misc.Launcher");
            Method method = launcherClass.getDeclaredMethod("getBootstrapClassPath", null);
            if(method != null){
                method.setAccessible(true);
                Object object = method.invoke(null, null);
                if(object != null){
                    Method methodGetUrls = object.getClass().getDeclaredMethod("getURLs", null);
                    if(methodGetUrls  != null){
                        methodGetUrls.setAccessible(true);
                        URL[] murlBoot = (URL[]) methodGetUrls.invoke(object, null);
                        printUrl(murlBoot);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

    }

    public static void printUrl(URL[] urls){
        for(URL url : urls){
            System.out.println(url);
        }
    }

    從運行結果可知：MainClass的ClassLoader是AppClassLoader，APPClassLoader的getParent字段返回的是ExtClassLoader，ExtClassLoader的getParent則是空。由於BootstrapClassLoader是C++實現的，因此此處經過sun.misc.Launcher此類來得到BootstrapClassLoader。值得注意的是APPClassLoader、ExtClassLoader都是sun.misc.Launcher的子類。

    從測試類的運行結果咱們能夠論證上圖的ClassLoader的加載路徑。AppClassLoader ClassPath所指定的目錄，ExtClassLoader jre/lib/ext/*，BootstrapClassLoader jre/rt.jar

/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java -Didea.launcher.port=7534 "-Didea.launcher.bin.path=/Applications/IntelliJ IDEA 15.app/Contents/bin" -Dfile.encoding=UTF-8 -classpath "/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/charsets.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/deploy.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/cldrdata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/dnsns.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jaccess.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jfxrt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/localedata.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/nashorn.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunec.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/zipfs.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/javaws.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jce.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jfr.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jfxswt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jsse.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/management-agent.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/plugin.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/rt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/ant-javafx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/dt.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/javafx-mx.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/jconsole.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/packager.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/sa-jdi.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/tools.jar:/Users/zcg/IdeaProjects/TestJava/out/production/TestJava:/Applications/IntelliJ IDEA 15.app/Contents/lib/idea_rt.jar" com.intellij.rt.execution.application.AppMain ClassLoaderTest
main classLoadersun.misc.Launcher$AppClassLoader@4b67cf4d
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/deploy.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/cldrdata.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/dnsns.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jaccess.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jfxrt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/localedata.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/nashorn.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunec.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/zipfs.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/javaws.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jfxswt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/management-agent.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/plugin.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/ant-javafx.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/dt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/javafx-mx.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/jconsole.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/packager.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/sa-jdi.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/lib/tools.jar
file:/Users/zcg/IdeaProjects/TestJava/out/production/TestJava/
file:/Applications/IntelliJ%20IDEA%2015.app/Contents/lib/idea_rt.jar

parent classLoadersun.misc.Launcher$ExtClassLoader@60e53b93
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/cldrdata.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/dnsns.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jaccess.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/jfxrt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/localedata.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/nashorn.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunec.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunjce_provider.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/sunpkcs11.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/ext/zipfs.jar
file:/System/Library/Java/Extensions/MRJToolkit.jar

BootstrapClassLoader
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/resources.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/rt.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/sunrsasign.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jsse.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jce.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/charsets.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/jfr.jar
file:/Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/classes

Process finished with exit code 0

三、ClassLoader的父委託機制

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;
        }
    }

此處是ClassLoader中的loadClass方法，java在類加載的時候會調用此方法。

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

    若是parent不爲空則調用parent的loadClass方法，下面咱們來看加載過來：首先調用AppClassLoader的loaderClass方法，parent不爲空則調用ExtClassLoader的loaderClass方法，由於BootstrapClassLoader是C++寫的因此ExtClassLoader的parent方法爲null因此在ExtClassLoader中會調用findBootstrapClassOrNull(name)方法。此方法交給BootstrapClass的loaderClass()方法來加載。

    BootstrapClassLoder的加載路徑jre/rt.jar若是沒有找到則由ExtClassLoader在jre/ext/*加載，仍是沒找到則由APPClassLoader在ClassPath中加載。這就是父委託加載機制，下面用一張圖來表示：