类加载器与单例

当使用不同的类加载器时，也会使单例失效，如下： 
单例为：  

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public final class Singleton{            private static final Singleton instance= new Singleton();            private Singleton(){                  System.out.println( "执行构造函数" );          System.out.println( "类加载器=" + this .getClass().getClassLoader());      }            public static Singleton getInstance(){          return instance;      }   }

自定义的类加载器为：  

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public class MyClassLoader extends ClassLoader{            private String name;      private String classPath;            public MyClassLoader(String name){          super ( null );          this .name = name;      }            @Override      protected Class<?> findClass(String name) throws ClassNotFoundException {          byte [] b=getClassBytes(name);          return this .defineClass(name, b, 0 ,b.length);      }        private byte [] getClassBytes(String name) {          String classFullPath=classPath+ "/" +name.replace( "." , "/" )+ ".class" ;          byte [] data= null ;          try {              FileInputStream fileInputStream= new FileInputStream(classFullPath);              ByteArrayOutputStream out= new ByteArrayOutputStream();              IOUtils.copy(fileInputStream,out);              data=out.toByteArray();          } catch (Exception e) {              e.printStackTrace();          }          return data;      }            public String getName() {          return name;      }        public void setName(String name) {          this .name = name;      }        public String getClassPath() {          return classPath;      }        public void setClassPath(String classPath) {          this .classPath = classPath;      }   }

测试案例如下：  

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public static void testClassLoader() throws Exception{          Singleton singleton=Singleton.getInstance();                    MyClassLoader myClassLoader= new MyClassLoader( "myClassLoader" );          myClassLoader.setClassPath( "D:/important" );          Class singletonClass=myClassLoader.findClass( "com.lg.design.singleton.hungry.Singleton" );          System.out.println( "singletonClass.getClassLoader() : " +singletonClass.getClassLoader());                    System.out.println( "Singleton.class==singletonClass : " +(Singleton. class ==singletonClass));          System.out.println( "Singleton.class.equals(singletonClass) : " +(Singleton. class .equals(singletonClass)));                    Constructor constructor1=Singleton. class .getDeclaredConstructor();          Constructor constructor2=Singleton. class .getDeclaredConstructor();          Constructor constructor3=singletonClass.getDeclaredConstructor();          System.out.println( "constructor1==constructor2 : " +(constructor1==constructor2));          System.out.println( "constructor1.equals(constructor2) : " +constructor1.equals(constructor2));          System.out.println( "constructor1==constructor : " +(constructor1==constructor3));          System.out.println( "constructor1.equals(constructor3) : " +constructor1.equals(constructor3));                    constructor1.setAccessible( true );          Object singleton1=constructor1.newInstance();          constructor3.setAccessible( true );          Object singleton3=constructor3.newInstance();                    System.out.println( "singleton : " +singleton);          System.out.println( "singleton1 : " +singleton1);          System.out.println( "singleton3 : " +singleton3);          System.out.println( "singleton1==singleton3 : " +(singleton1==singleton3));      }

输出结果为：  

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执行构造函数 类加载器=sun.misc.Launcher$AppClassLoader @417470d0 singletonClass.getClassLoader() : com.lg.design.singleton.hungry.MyClassLoader @470d1f30 Singleton. class ==singletonClass : false Singleton. class .equals(singletonClass) : false constructor1==constructor2 : false constructor1.equals(constructor2) : true constructor1==constructor : false constructor1.equals(constructor3) : false 执行构造函数 类加载器=sun.misc.Launcher$AppClassLoader @417470d0 singleton : com.lg.design.singleton.hungry.Singleton @77e3cabd singleton1 : com.lg.design.singleton.hungry.Singleton @c137bc9 singleton3 : com.lg.design.singleton.hungry.Singleton @5323cf50 singleton1==singleton3 : false

咱们慢慢来看这些信息。 
1 Singleton.class与singletonClass 
前者是系统类加载器加载器的，后者是我们自定义的类加载器加载的，虽然他们的字节码相同，但由不同的类加载器加载后就是不同的类了，所以两者的==和eaquals都为false。 
2 constructor1、constructor2、constructor3 
constructor1、constructor2都是通过调用Singleton.class.getDeclaredConstructor()得来的，但是两者并不是同一个对象，他们的==为false，equals为true。看getDeclaredConstructor源码就可以理解：  

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private Constructor<T> getConstructor0(Class<?>[] parameterTypes,                                          int which) throws NoSuchMethodException      {          Constructor<T>[] constructors = privateGetDeclaredConstructors((which == Member.PUBLIC));          for (Constructor<T> constructor : constructors) {              if (arrayContentsEq(parameterTypes,                                  constructor.getParameterTypes())) {                  //这里在获取构造器的时候就是用的复制                  return getReflectionFactory().copyConstructor(constructor);              }          }          throw new NoSuchMethodException(getName() + ".<init>" + argumentTypesToString(parameterTypes));      }

再看构造器的eequals方法  

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public boolean equals(Object obj) {          if (obj != null && obj instanceof Constructor) {              Constructor<?> other = (Constructor<?>)obj;              if (getDeclaringClass() == other.getDeclaringClass()) {                  /* Avoid unnecessary cloning */                  Class<?>[] params1 = parameterTypes;                  Class<?>[] params2 = other.parameterTypes;                  if (params1.length == params2.length) {                      for ( int i = 0 ; i < params1.length; i++) {                          if (params1[i] != params2[i])                              return false ;                      }                      return true ;                  }              }          }          return false ;      }

先通过比较是否是同一个类的构造器，然后再比较他们的参数是否一致，所以constructor1和constructor2的equals方法为true。对于constructor3和constructor1、constructor2，他们所属的类就是不一样的，即getDeclaringClass() == other.getDeclaringClass()为false。 

3 singleton1和singleton3 
singleton1是由constructor1构造器通过反射生成的对象，constructor3是通过constructor3构造器通过反射生成的对象，这些对象肯定都不是同一个对象。我有个疑问就是：通过constructor1.newInstance()会去执行Singleton的无参构造函数，打印出 

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1 2	执行构造函数 类加载器=sun.misc.Launcher$AppClassLoader @417470d0

然而执行constructor3.newInstance()却并没有打印出无参构造函数中的信息，这背后的原理希望你们能帮我解答。 
有关类加载器的内容，请见后续文章