原理

JVM在运行时才动态加载类或者调用方法以及访问属性,不需要事先(比如编译时)知道运行对象是什么。

实现

1. 反射获取类实例 Class.forName(“xxx”);

首先调用了java.lang.Class的静态方法，获取类信息！
注意：forName()反射获取类信息，并没有将实现留给了java,而是交给了jvm去加载！
主要是先获取 ClassLoader, 然后调用 native 方法，获取信息，加载类则是回调 入参ClassLoader 进类加载！

  1. @CallerSensitive
  2. public static Class<?> forName(String className)  throws ClassNotFoundException {
  3.     // 先通过反射，获取调用进来的类信息，从而获取当前的 classLoader
  4.     Class<?> caller = Reflection.getCallerClass();
  5.     // 调用native方法进行获取class信息
  6.     return forName0(className, true, ClassLoader.getClassLoader(caller), caller);
  7. }

2. ClassLoader 进类加载

  1. protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException
  2.     {
  3.         // 获取锁
  4.         synchronized (getClassLoadingLock(name)) {
  5.             // 如果已经加载了的话，就不用再加载了
  6.             Class<?> c = findLoadedClass(name);
  7.             if (c == null) {
  8.                 long t0 = System.nanoTime();
  9.                 try {
  10.                     // 双亲委托加载
  11.                     if (parent != null) {
  12.                         c = parent.loadClass(name, false);
  13.                     } else {
  14.                         c = findBootstrapClassOrNull(name);
  15.                     }
  16.                 } catch (ClassNotFoundException e) {
  17.                     // ClassNotFoundException thrown if class not found
  18.                     // from the non-null parent class loader
  19.                 }
  21.                 if (c == null) {
  22.                     // If still not found, then invoke findClass in order
  23.                     // to find the class.
  24.                     long t1 = System.nanoTime();
  25.                     c = findClass(name);
  27.                     // this is the defining class loader; record the stats
  28.                     sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
  29.                     sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
  30.                     sun.misc.PerfCounter.getFindClasses().increment();
  31.                 }
  32.             }
  33.             if (resolve) {
  34.                 resolveClass(c);
  35.             }
  36.             return c;
  37.         }
  38.     }
  40. private final ConcurrentHashMap<String, Object> parallelLockMap;
  42. // 获取类加载锁
  43. protected Object getClassLoadingLock(String className) {
  44.         Object lock = this;
  45.         if (parallelLockMap != null) {
  46.             Object newLock = new Object();
  47.             // 使用 ConcurrentHashMap来保存锁
  48.             lock = parallelLockMap.putIfAbsent(className, newLock);
  49.             if (lock == null) {
  50.                 lock = newLock;
  51.             }
  52.         }
  53.         return lock;
  54. }
  56.     protected final Class<?> findLoadedClass(String name) {
  57.         if (!checkName(name))
  58.             return null;
  59.         return findLoadedClass0(name);
  60.     }
  62.     private boolean checkName(String name) {
  63.         if ((name == null) || (name.length() == 0))
  64.             return true;
  65.         if ((name.indexOf('/') != -1)
  66.             || (!VM.allowArraySyntax() && (name.charAt(0) == '[')))
  67.             return false;
  68.         return true;
  69.     }
  71.     private native final Class<?> findLoadedClass0(String name);

3. newInstance()

  1. @CallerSensitive
  2. public T newInstance(Object ... initargs)
  3.         throws InstantiationException, IllegalAccessException,IllegalArgumentException, InvocationTargetException
  4. {
  5.     if (!override) {
  6.         if (!Reflection.quickCheckMemberAccess(clazz, modifiers)) {
  7.             Class<?> caller = Reflection.getCallerClass();
  8.             checkAccess(caller, clazz, null, modifiers);
  9.         }
  10.     }
  11.     if ((clazz.getModifiers() & Modifier.ENUM) != 0)
  12.                 throw new IllegalArgumentException("Cannot reflectively create enum objects");
  13.     ConstructorAccessor ca = constructorAccessor;
  14.     // read volatile
  15.     if (ca == null) {
  16.         ca = acquireConstructorAccessor();
  17.     }
  18.     @SuppressWarnings("unchecked")
  19.             T inst = (T) ca.newInstance(initargs);
  20.     return inst;
  21. }
  23. private volatile ConstructorAccessor constructorAccessor;

用处

  • 实现自定义注解（AOP）
  • 实现动态代理（DI）
  • 可以访问一些私有变量和属性