读《深入理解Java虚拟机》有感——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.ClassLoader
20       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 ClassNotFoundException
27     {
28         synchronized (getClassLoadingLock(name)) {
29             // First, check if the class has already been loaded
30             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 found
41                     // from the non-null parent class loader
42                 }
43 
44                 if (c == null) {
45                     // If still not found, then invoke findClass in order
46                     // to find the class.
47                     long t1 = System.nanoTime();
48                     c = findClass(name);
49 
50                     // this is the defining class loader; record the stats
51                     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 classLoader
77       *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)