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| ## ClassLoader | ||
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| + Bootstrap Loader - 负责加载系统类 | ||
| + ExtClassLoader - 负责加载扩展类 | ||
| + AppClassLoader - 负责加载应用类 | ||
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| 当执行 java ***.class 的时: | ||
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|   java.exe 会帮助我们找到 JRE ，接着找到位于 JRE 内部的 jvm.dll ，这才是真正的 Java 虚拟机器 , 最后加载动态库，激活 Java 虚拟机器。虚拟机器激活以后，会先做一些初始化的动作，比如说读取系统参数等。一旦初始化动作完成之后，就会产生第一个类加载器―― Bootstrap Loader | ||
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|   Bootstrap Loader 是由 C++ 所撰写而成，这个 Bootstrap Loader 所做的初始工作中，除了一些基本的初始化动作之外，最重要的就是加载 Launcher.java 之中的 ExtClassLoader ，并设定其 Parent 为 null ，代表其父加载器为 BootstrapLoader 。（Launcher.java在rt.jar）然后 Bootstrap Loader 再要求加载 Launcher.java 之中的 AppClassLoader ，并设定其 Parent 为之前产生的 ExtClassLoader 实体。这两个加载器都是以静态类的形式存在的。 | ||
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|   这里要注意的是： Launcher$ExtClassLoader.class 与 Launcher$AppClassLoader.class 都是由 Bootstrap Loader 所加载，所以 Parent 和由哪个类加载器加载没有关系。 | ||
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| 根据指定的name名称装载class。默认的实现中，搜索顺序如下： | ||
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| 1. 调用 `findLoadedClass(String)`检查是否已加载 | ||
| 2. 调用 父类的`loadClass(String)` 方法，如果parent==null,则使用内置装载器Bootstrap classloader | ||
| 3. 调用 findClass(Stirng)查找类 | ||
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| 如果在上述步骤中，已经发现class，并且resolve标记为true，本方法将调用resolveClass(Class)，Class是传入的Class文件 | ||
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| ClassLoader 的子类，鼓励覆盖@Override $findClass(String)$,而不是@Override loadClass(String) | ||
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| 在没有被重写的情况下，此方法 使用$getClassLoadingLock$ 保证整个类装载过程的同步 | ||
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| ```java {.line-numbers} | ||
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| /** | ||
| * @param name | ||
| * 类名 | ||
| * @param resolve | ||
| * true: resolve the class | ||
| * @return Class | ||
| * @throws ClassNotFoundException | ||
| * If the class could not be found | ||
| */ | ||
| protected Class<?> loadClass(String name, boolean resolve) | ||
| throws ClassNotFoundException | ||
| { | ||
| //同步锁，保证整个类装载过程的同步 | ||
| synchronized (getClassLoadingLock(name)) { | ||
| // 1. 检查类是否被装载 | ||
| Class<?> c = findLoadedClass(name); | ||
| if (c == null) { | ||
| long t0 = System.nanoTime(); | ||
| try { | ||
| //向父 loader委派 | ||
| 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); | ||
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| // 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; | ||
| } | ||
| } | ||
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| ``` | ||
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| 使用给定的类加载器， 返回与 name 关联的class或interface 的Class 对象. | ||
| 给定类的完全限定名(getName 返回的格式)，此方法尝试查找、加载和链接类或接口。指定的类。使用指定的加载器加载类或接口。如果参数为 null, 通过 bootstrap 加载，只有当initialize==true&&类没有被初始化过，才会初始化类。 | ||
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| + 如果name 传入一个基本类型或者void， 将尝试在 未命名的package的包里定位一个自定义类，因此这个方法不能用户获取任何基础类型和void的Class | ||
| + 如果是一个数组 | ||
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| ```java {.line-numbers} | ||
| /** | ||
| * Returns the {@code Class} object associated with the class or | ||
| * interface with the given string name, using the given class loader. | ||
| * Given the fully qualified name for a class or interface (in the same | ||
| * format returned by {@code getName}) this method attempts to | ||
| * locate, load, and link the class or interface. The specified class | ||
| * loader is used to load the class or interface. If the parameter | ||
| * {@code loader} is null, the class is loaded through the bootstrap | ||
| * class loader. The class is initialized only if the | ||
| * {@code initialize} parameter is {@code true} and if it has | ||
| * not been initialized earlier. | ||
| * | ||
| * <p> If {@code name} denotes a primitive type or void, an attempt | ||
| * will be made to locate a user-defined class in the unnamed package whose | ||
| * name is {@code name}. Therefore, this method cannot be used to | ||
| * obtain any of the {@code Class} objects representing primitive | ||
| * types or void. | ||
| * | ||
| * <p> If {@code name} denotes an array class, the component type of | ||
| * the array class is loaded but not initialized. | ||
| * | ||
| * <p> For example, in an instance method the expression: | ||
| * | ||
| * <blockquote> | ||
| * {@code Class.forName("Foo")} | ||
| * </blockquote> | ||
| * | ||
| * is equivalent to: | ||
| * | ||
| * <blockquote> | ||
| * {@code Class.forName("Foo", true, this.getClass().getClassLoader())} | ||
| * </blockquote> | ||
| * | ||
| * Note that this method throws errors related to loading, linking or | ||
| * initializing as specified in Sections 12.2, 12.3 and 12.4 of <em>The | ||
| * Java Language Specification</em>. | ||
| * Note that this method does not check whether the requested class | ||
| * is accessible to its caller. | ||
| * | ||
| * <p> If the {@code loader} is {@code null}, and a security | ||
| * manager is present, and the caller's class loader is not null, then this | ||
| * method calls the security manager's {@code checkPermission} method | ||
| * with a {@code RuntimePermission("getClassLoader")} permission to | ||
| * ensure it's ok to access the bootstrap class loader. | ||
| * | ||
| * @param name fully qualified name of the desired class | ||
| * @param initialize if {@code true} the class will be initialized. | ||
| * See Section 12.4 of <em>The Java Language Specification</em>. | ||
| * @param loader class loader from which the class must be loaded | ||
| * @return class object representing the desired class | ||
| * | ||
| * @exception LinkageError if the linkage fails | ||
| * @exception ExceptionInInitializerError if the initialization provoked | ||
| * by this method fails | ||
| * @exception ClassNotFoundException if the class cannot be located by | ||
| * the specified class loader | ||
| * | ||
| * @see java.lang.Class#forName(String) | ||
| * @see java.lang.ClassLoader | ||
| * @since 1.2 | ||
| */ | ||
| @CallerSensitive | ||
| public static Class<?> forName(String name, boolean initialize, | ||
| ClassLoader loader) | ||
| throws ClassNotFoundException | ||
| { | ||
| Class<?> caller = null; | ||
| SecurityManager sm = System.getSecurityManager(); | ||
| if (sm != null) { | ||
| // Reflective call to get caller class is only needed if a security manager | ||
| // is present. Avoid the overhead of making this call otherwise. | ||
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| // | ||
| caller = Reflection.getCallerClass(); | ||
| if (sun.misc.VM.isSystemDomainLoader(loader)) { | ||
| ClassLoader ccl = ClassLoader.getClassLoader(caller); | ||
| if (!sun.misc.VM.isSystemDomainLoader(ccl)) { | ||
| sm.checkPermission( | ||
| SecurityConstants.GET_CLASSLOADER_PERMISSION); | ||
| } | ||
| } | ||
| } | ||
| return forName0(name, initialize, loader, caller); | ||
| } | ||
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| ``` | ||
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| ### Tomcat中的类加载器 | ||
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| 在Tomcat目录结构中，有三组目录（“/common/*”,“/server/*”和“shared/*”）可以存放公用Java类库， | ||
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| 此外还有第四组Web应用程序自身的目录“/WEB-INF/*”，把java类库放置在这些目录中的含义分别是： | ||
| + common：类库可被Tomcat和所有的Web应用程序共同使用。 | ||
| + server：类库可被Tomcat使用，但对所有的Web应用程序都不可见。 | ||
| + shared：类库可被所有的Web应用程序共同使用，但对Tomcat自己不可见。 | ||
| + /WebApp/WEB-INF：类库仅仅可以被此Web应用程序使用，对Tomcat和其他Web应用程序都不可见。 | ||
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|  | ||
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| JVM区域总体分两类，heap区和非heap区。heap区又分：Eden Space（伊甸园）、Survivor Space(幸存者区)、Tenured Gen（老年代-养老区）。 非heap区又分：Code Cache(代码缓存区)、Perm Gen（永久代）、Jvm Stack(java虚拟机栈)、Local Method Statck(本地方法栈)。 | ||
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