读《深入理解Java虚拟机》随感——第二部分:虚拟机类加载机制

 1 <一>java.lang.Object 2 1.getClass() 3 public final native Class<?> getClass();    //拿到运行时的Class对象【通过本地方法】 4 /** 5  *例子：class-Test>main>  6  *                    Class c =Person.getClass(); 7  */ 8      /** 9        *实现：  10        *1）虚拟机启动11        *2)虚拟机类加载——Test.class————加载阶段：方法区>外部接口>new java.lang.Class  //Class<?>     12        *3)虚拟机类加载——java.lang.Object————【Test加载阶段】触发13        *3)虚拟机类加载——Person.class    ————【Test加载阶段】触发14        *4)应用程序启动15        *5）调用java.lang.ClassLoader>xxx1()、xxx2().....——返回运行时<Person>Class对象16        */17 18 19 <二>java。lang.ClassLoader20       1.loadClass(String name, boolean resolve)  /**加载指定名称（包括包名）的二进制类型，同时指定是否解析*/21         loadClass(String name)   22     protected Class<?> findClass(String name) throws ClassNotFoundException {   //空方法23         throw new ClassNotFoundException(name);24     }25     protected Class<?> loadClass(String name, boolean resolve)    //拿到类加载器【通过本地方法】26         throws ClassNotFoundException27     {28         synchronized (getClassLoadingLock(name)) {29             // First, check if the class has already been loaded30             Class c = findLoadedClass(name);31             if (c == null) {32                 long t0 = System.nanoTime();33                 try {34                     if (parent != null) {35                         c = parent.loadClass(name, false);36                     } else {37                         c = findBootstrapClassOrNull(name);38                     }39                 } catch (ClassNotFoundException e) {40                     // ClassNotFoundException thrown if class not found41                     // from the non-null parent class loader42                 }43 44                 if (c == null) {45                     // If still not found, then invoke findClass in order46                     // to find the class.47                     long t1 = System.nanoTime();48                     c = findClass(name);49 50                     // this is the defining class loader; record the stats51                     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);52                     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);53                     sun.misc.PerfCounter.getFindClasses().increment();54                 }55             }56             if (resolve) {57                 resolveClass(c);58             }59             return c;60         }61     }62     63     64     65     66 <三>java.lang.Class   67      1.getClassLoader()68 /**例子：class-Test>main>69   *                    Object o =new Object();70   *                       System.out.println(o.getClass().getClassLoader());71                                                                    结果>java.lang.NullPointerException   72   */                          73      /**74       *Returns the class loader for the class.  75       *This method will return null in such implementations if this class was loaded by the bootstrap class loader. 76       *如果没有指定classLoader就默认返回bootstrap classLoader(启动类加载器)，因为这个bootstrap classLoader77       *用户拿不到实例,所以返回null表示返回的是bootstrap classLoader     78       */79 native ClassLoader getClassLoader0();      //拿到类加载器【通过本地方法】80 public ClassLoader getClassLoader() {81         ClassLoader cl = getClassLoader0();82         if (cl == null)83             return null;84         SecurityManager sm = System.getSecurityManager();85         if (sm != null) {86             ClassLoader.checkClassLoaderPermission(cl, Reflection.getCallerClass());87         }88         return cl;89     }
         2.指定类加载器加载类
           Class.forName(name, initialize, loader)