类加载流程图

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1.创建引导类加载器（C语言实现）（bootstrapClassLoader）
2.创建JVM启动器
3.实例化启动器过程中，初始化extClassLoader、AppClassLoader。

一、JVM启动器结构预览

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扩展与应用类加载器都是启动器内部类

  1.     package sun.misc;
  2.     public class Launcher {
  3.         private static URLStreamHandlerFactory factory = new Launcher.Factory();
  4.         //加载是实例化
  5.         private static Launcher launcher = new Launcher();
  6.         private static String bootClassPath = System.getProperty("sun.boot.class.path");
  7.         private ClassLoader loader;
  8.         private static URLStreamHandler fileHandler;
  10.         public static Launcher getLauncher() {
  11.             return launcher;
  12.         }
  14.         public Launcher() {
  15.             Launcher.ExtClassLoader var1;
  16.             try {
  17.                 //获取扩展类加载器对象。（扩展类加载器，是Launcher的一个内部类）
  18.                 var1 = Launcher.ExtClassLoader.getExtClassLoader();
  19.             } catch (IOException var10) {
  20.                 throw new InternalError("Could not create extension class loader", var10);
  21.             }
  23.             try {
  24.                 //扩展类加载器作为入参，最终为AppClassLoader中parent属性赋值，即标识父加载器
  25.                 //（应用类加载器，是Launcher的一个内部类）
  26.                 this.loader = Launcher.AppClassLoader.getAppClassLoader(var1);
  27.             } catch (IOException var9) {
  28.                 throw new InternalError("Could not create application class loader", var9);
  29.             }
  31.             Thread.currentThread().setContextClassLoader(this.loader);
  32.             String var2 = System.getProperty("java.security.manager");
  33.             if (var2 != null) {
  34.                 SecurityManager var3 = null;
  35.                 if (!"".equals(var2) && !"default".equals(var2)) {
  36.                     try {
  37.                         var3 = (SecurityManager) this.loader.loadClass(var2).newInstance();
  38.                     } catch (IllegalAccessException var5) {
  39.                         ;
  40.                     } catch (InstantiationException var6) {
  41.                         ;
  42.                     } catch (ClassNotFoundException var7) {
  43.                         ;
  44.                     } catch (ClassCastException var8) {
  45.                         ;
  46.                     }
  47.                 } else {
  48.                     var3 = new SecurityManager();
  49.                 }
  51.                 if (var3 == null) {
  52.                     throw new InternalError("Could not create SecurityManager: " + var2);
  53.                 }
  55.                 System.setSecurityManager(var3);
  56.             }
  58.         }
  60.         /***
  61.          *获取应用类加载器appClassLoader
  62.          **/
  63.         public ClassLoader getClassLoader() {
  64.             return this.loader;
  65.         }
  67.         public static URLClassPath getBootstrapClassPath() {
  68.             return Launcher.BootClassPathHolder.bcp;
  69.         }
  71.         private static File[] getClassPath(String var0) {
  72.             File[] var1;
  73.             if (var0 != null) {
  74.                 int var2 = 0;
  75.                 int var3 = 1;
  76.                 boolean var4 = false;
  78.                 int var5;
  79.                 int var7;
  80.                 for (var5 = 0; (var7 = var0.indexOf(File.pathSeparator, var5)) != -1; var5 = var7 + 1) {
  81.                     ++var3;
  82.                 }
  84.                 var1 = new File[var3];
  85.                 var4 = false;
  87.                 for (var5 = 0; (var7 = var0.indexOf(File.pathSeparator, var5)) != -1; var5 = var7 + 1) {
  88.                     if (var7 - var5 > 0) {
  89.                         var1[var2++] = new File(var0.substring(var5, var7));
  90.                     } else {
  91.                         var1[var2++] = new File(".");
  92.                     }
  93.                 }
  95.                 if (var5 < var0.length()) {
  96.                     var1[var2++] = new File(var0.substring(var5));
  97.                 } else {
  98.                     var1[var2++] = new File(".");
  99.                 }
  101.                 if (var2 != var3) {
  102.                     File[] var6 = new File[var2];
  103.                     System.arraycopy(var1, 0, var6, 0, var2);
  104.                     var1 = var6;
  105.                 }
  106.             } else {
  107.                 var1 = new File[0];
  108.             }
  110.             return var1;
  111.         }
  114.         static class AppClassLoader extends URLClassLoader {
  115.             final URLClassPath ucp = SharedSecrets.getJavaNetAccess().getURLClassPath(this);
  117.             public static ClassLoader getAppClassLoader(final ClassLoader var0) throws IOException {
  118.                 final String var1 = System.getProperty("java.class.path");
  119.                 final File[] var2 = var1 == null ? new File[0] : Launcher.getClassPath(var1);
  120.                 return (ClassLoader) AccessController.doPrivileged(new PrivilegedAction<Launcher.AppClassLoader>() {
  121.                     public Launcher.AppClassLoader run() {
  122.                         URL[] var1x = var1 == null ? new URL[0] : Launcher.pathToURLs(var2);
  123.                         return new Launcher.AppClassLoader(var1x, var0);
  124.                     }
  125.                 });
  126.             }
  128.             AppClassLoader(URL[] var1, ClassLoader var2) {
  129.                 super(var1, var2, Launcher.factory);
  130.                 this.ucp.initLookupCache(this);
  131.             }
  133.             public Class<?> loadClass(String var1, boolean var2) throws ClassNotFoundException {
  134.                 int var3 = var1.lastIndexOf(46);
  135.                 if (var3 != -1) {
  136.                     SecurityManager var4 = System.getSecurityManager();
  137.                     if (var4 != null) {
  138.                         var4.checkPackageAccess(var1.substring(0, var3));
  139.                     }
  140.                 }
  142.                 if (this.ucp.knownToNotExist(var1)) {
  143.                     Class var5 = this.findLoadedClass(var1);
  144.                     if (var5 != null) {
  145.                         if (var2) {
  146.                             this.resolveClass(var5);
  147.                         }
  149.                         return var5;
  150.                     } else {
  151.                         throw new ClassNotFoundException(var1);
  152.                     }
  153.                 } else {
  154.                     return super.loadClass(var1, var2);
  155.                 }
  156.             }
  158.             protected PermissionCollection getPermissions(CodeSource var1) {
  159.                 PermissionCollection var2 = super.getPermissions(var1);
  160.                 var2.add(new RuntimePermission("exitVM"));
  161.                 return var2;
  162.             }
  164.             private void appendToClassPathForInstrumentation(String var1) {
  165.                 assert Thread.holdsLock(this);
  167.                 super.addURL(Launcher.getFileURL(new File(var1)));
  168.             }
  170.             private static AccessControlContext getContext(File[] var0) throws MalformedURLException {
  171.                 PathPermissions var1 = new PathPermissions(var0);
  172.                 ProtectionDomain var2 = new ProtectionDomain(new CodeSource(var1.getCodeBase(), (Certificate[]) null), var1);
  173.                 AccessControlContext var3 = new AccessControlContext(new ProtectionDomain[]{var2});
  174.                 return var3;
  175.             }
  177.             static {
  178.                 ClassLoader.registerAsParallelCapable();
  179.             }
  180.         }
  182.         private static class BootClassPathHolder {
  183.             static final URLClassPath bcp;
  185.             private BootClassPathHolder() {
  186.             }
  188.             static {
  189.                 URL[] var0;
  190.                 if (Launcher.bootClassPath != null) {
  191.                     var0 = (URL[]) AccessController.doPrivileged(new PrivilegedAction<URL[]>() {
  192.                         public URL[] run() {
  193.                             File[] var1 = Launcher.getClassPath(Launcher.bootClassPath);
  194.                             int var2 = var1.length;
  195.                             HashSet var3 = new HashSet();
  197.                             for (int var4 = 0; var4 < var2; ++var4) {
  198.                                 File var5 = var1[var4];
  199.                                 if (!var5.isDirectory()) {
  200.                                     var5 = var5.getParentFile();
  201.                                 }
  203.                                 if (var5 != null && var3.add(var5)) {
  204.                                     MetaIndex.registerDirectory(var5);
  205.                                 }
  206.                             }
  208.                             return Launcher.pathToURLs(var1);
  209.                         }
  210.                     });
  211.                 } else {
  212.                     var0 = new URL[0];
  213.                 }
  215.                 bcp = new URLClassPath(var0, Launcher.factory, (AccessControlContext) null);
  216.                 bcp.initLookupCache((ClassLoader) null);
  217.             }
  218.         }
  220.         static class ExtClassLoader extends URLClassLoader {
  221.             public static Launcher.ExtClassLoader getExtClassLoader() throws IOException {
  222.                 final File[] var0 = getExtDirs();
  224.                 try {
  225.                     return (Launcher.ExtClassLoader) AccessController.doPrivileged(new PrivilegedExceptionAction<Launcher.ExtClassLoader>() {
  226.                         public Launcher.ExtClassLoader run() throws IOException {
  227.                             int var1 = var0.length;
  229.                             for (int var2 = 0; var2 < var1; ++var2) {
  230.                                 MetaIndex.registerDirectory(var0[var2]);
  231.                             }
  233.                             return new Launcher.ExtClassLoader(var0);
  234.                         }
  235.                     });
  236.                 } catch (PrivilegedActionException var2) {
  237.                     throw (IOException) var2.getException();
  238.                 }
  239.             }
  241.             void addExtURL(URL var1) {
  242.                 super.addURL(var1);
  243.             }
  245.             //因为bootstrapClassLoader是C++实现，没有具体java对象，故ExtClassLoader对象parent属性为空。
  246.             public ExtClassLoader(File[] var1) throws IOException {
  247.                 super(getExtURLs(var1), (ClassLoader) null, Launcher.factory);
  248.                 SharedSecrets.getJavaNetAccess().getURLClassPath(this).initLookupCache(this);
  249.             }
  251.             private static File[] getExtDirs() {
  252.                 String var0 = System.getProperty("java.ext.dirs");
  253.                 File[] var1;
  254.                 if (var0 != null) {
  255.                     StringTokenizer var2 = new StringTokenizer(var0, File.pathSeparator);
  256.                     int var3 = var2.countTokens();
  257.                     var1 = new File[var3];
  259.                     for (int var4 = 0; var4 < var3; ++var4) {
  260.                         var1[var4] = new File(var2.nextToken());
  261.                     }
  262.                 } else {
  263.                     var1 = new File[0];
  264.                 }
  266.                 return var1;
  267.             }
  269.             private static URL[] getExtURLs(File[] var0) throws IOException {
  270.                 Vector var1 = new Vector();
  272.                 for (int var2 = 0; var2 < var0.length; ++var2) {
  273.                     String[] var3 = var0[var2].list();
  274.                     if (var3 != null) {
  275.                         for (int var4 = 0; var4 < var3.length; ++var4) {
  276.                             if (!var3[var4].equals("meta-index")) {
  277.                                 File var5 = new File(var0[var2], var3[var4]);
  278.                                 var1.add(Launcher.getFileURL(var5));
  279.                             }
  280.                         }
  281.                     }
  282.                 }
  284.                 URL[] var6 = new URL[var1.size()];
  285.                 var1.copyInto(var6);
  286.                 return var6;
  287.             }
  289.             public String findLibrary(String var1) {
  290.                 var1 = System.mapLibraryName(var1);
  291.                 URL[] var2 = super.getURLs();
  292.                 File var3 = null;
  294.                 for (int var4 = 0; var4 < var2.length; ++var4) {
  295.                     URI var5;
  296.                     try {
  297.                         var5 = var2[var4].toURI();
  298.                     } catch (URISyntaxException var9) {
  299.                         continue;
  300.                     }
  302.                     File var6 = (new File(var5)).getParentFile();
  303.                     if (var6 != null && !var6.equals(var3)) {
  304.                         String var7 = VM.getSavedProperty("os.arch");
  305.                         File var8;
  306.                         if (var7 != null) {
  307.                             var8 = new File(new File(var6, var7), var1);
  308.                             if (var8.exists()) {
  309.                                 return var8.getAbsolutePath();
  310.                             }
  311.                         }
  313.                         var8 = new File(var6, var1);
  314.                         if (var8.exists()) {
  315.                             return var8.getAbsolutePath();
  316.                         }
  317.                     }
  319.                     var3 = var6;
  320.                 }
  322.                 return null;
  323.             }
  325.             private static AccessControlContext getContext(File[] var0) throws IOException {
  326.                 PathPermissions var1 = new PathPermissions(var0);
  327.                 ProtectionDomain var2 = new ProtectionDomain(new CodeSource(var1.getCodeBase(), (Certificate[]) null), var1);
  328.                 AccessControlContext var3 = new AccessControlContext(new ProtectionDomain[]{var2});
  329.                 return var3;
  330.             }
  332.             static {
  333.                 ClassLoader.registerAsParallelCapable();
  334.             }
  335.         }
  337.         private static class Factory implements URLStreamHandlerFactory {
  338.             private static String PREFIX = "sun.net.www.protocol";
  340.             private Factory() {
  341.             }
  343.             public URLStreamHandler createURLStreamHandler(String var1) {
  344.                 String var2 = PREFIX + "." + var1 + ".Handler";
  346.                 try {
  347.                     Class var3 = Class.forName(var2);
  348.                     return (URLStreamHandler) var3.newInstance();
  349.                 } catch (ReflectiveOperationException var4) {
  350.                     throw new InternalError("could not load " + var1 + "system protocol handler", var4);
  351.                 }
  352.             }
  353.         }
  354.     }

二、自定义类加载器

打破双亲委派，改写ClassLoader中查找父类加载器的逻辑即可，避免加载自定义类时，被委派给应用类加载器。（注意jdk核心代码需要，仍然使用双亲委派，不能使用自定义类加载器加载jdk核心包）