java反射学习笔记-core java

1.反射概述

反射库是个强大的工具库，大量用于javaBean中。能够分析类能力的程序都被称为反射。

作用有：

1.分析类的结构。（后面实现了一个类结构分析器）

2.在运行时查看对象。（不管是公共还是私有的）后面实现了一个超级攻击类，万能toString（）

3.实现数组的操作代码

4.利用method对象，很像C++的函数指针。（可以调用类似于单例模式的私有化的构造器）

2.利用反射分析类的能力

利用反射，分析类的构造器，私有变量，方法等。

/** * 源码结构查看器（这里只实现了查看构造函数= =！） * java.lang.Double * java.io.FileInputStream * @param args */public static void main(String[] args) throws Exception{Scanner scanner = new Scanner(System.in);String className = scanner.next();String tmpString = "";Class c = Class.forName(className);Class[] interfaces = c.getInterfaces();Class cp = c.getSuperclass();System.out.println("public class "+className+" extends "+cp.toString()+" implements ");for(Class tmp:interfaces){tmpString+=tmp.toString()+" ";}System.out.println(tmpString);System.out.println("{");Constructor[] cons = c.getConstructors();tmpString = "";//输出构造函数for(Constructor tmp:cons){String modi = Modifier.toString(tmp.getModifiers());for(Class pt:tmp.getParameterTypes()){tmpString+=pt.toString()+",";}tmpString = tmpString.replace("class ", "");System.out.println(""+modi+" "+tmp.getName()+"("+tmpString.substring(0, tmpString.length()-1)+")");}//System.out.println("}");}

3.运行时使用反射分析对象

利用反射可以访问类的私有变量

Field.setAccessible(true);

这个方法是Accessible的一个方法，这是Field， Method ，Constructor的超类

public class SuperToStringUtils {public static void main(String[] args) throws Exception {Student c = new Student(1001,"隔壁老王","软件工程");System.out.println(SuperToStringUtils.toString(c));}public static <T> String toString(Object c) throws Exception{String out = "";Class cl = c.getClass();Field[] fields = cl.getDeclaredFields() ;if(cl==String.class)return (String) c;for(Field f:fields){f.setAccessible(true);out+="属性"+f.getName()+"的值为:"+f.get(c)+"";}return out;}}class Student{private int id;private String name;private String classname;private java.io.File File = new java.io.File("./file.tmp");private Date date = new Date(System.currentTimeMillis());public Student(int id, String name, String classname) {super();this.id = id;this.name = name;this.classname = classname;}public Student() {super();}}

4.调用私有的方法或者构造函数

下面举一个单例模式中的一个问题，哪种单例的实现形式是绝对的单例

public class NotSingletion {/** * 由此可见~~单例模式也不是绝对的，除了第三个用枚举类实现的，老夫真的没辙了 * @param args * @throws Exception */public static void main(String[] args) throws Exception {Constructor constructor = (Constructor<SingletionA>) SingletionA.class.getDeclaredConstructors()[0];constructor.setAccessible(true);System.out.println("No1 Instance:"+SingletionA.getInstance());System.out.println("No2 Instance:"+constructor.newInstance());System.out.println("No3 Instance:"+constructor.newInstance());System.out.println("==========================================");constructor = (Constructor<SingletionB>) SingletionB.class.getDeclaredConstructors()[0];constructor.setAccessible(true);System.out.println("No1 Instance:"+SingletionA.getInstance());System.out.println("No2 Instance:"+constructor.newInstance());System.out.println("No3 Instance:"+constructor.newInstance());System.out.println("==========================================");constructor = (Constructor<SingletionC>) SingletionC.class.getDeclaredConstructors()[0];constructor.setAccessible(true);System.out.println("No1 Instance:"+SingletionA.getInstance());System.out.println("No2 Instance:"+constructor.newInstance());System.out.println("No3 Instance:"+constructor.newInstance());}}class SingletionA{        private SingletionA(){                        }          private static class lazyHolder{              private static SingletionA init = new SingletionA();          }          public static SingletionA getInstance(){              return lazyHolder.init;      }  }class SingletionB{private static SingletionB singletion = new SingletionB();private SingletionB(){}public static SingletionB getInstance(){if(singletion==null){  synchronized(SingletionB.class)  {         singletion = new SingletionB();  }  }return singletion;}}enum SingletionC{instance{public void doing(){System.out.println("123456");}};public abstract void doing();}

No1 Instance:day20150730.SingletionA@2f9ee1ac
No2 Instance:day20150730.SingletionA@3fbefab0
No3 Instance:day20150730.SingletionA@133c5982
==========================================
No1 Instance:day20150730.SingletionA@2f9ee1ac
No2 Instance:day20150730.SingletionB@3d4b7453
No3 Instance:day20150730.SingletionB@24c21495
==========================================
No1 Instance:day20150730.SingletionA@2f9ee1ac
Exception in thread "main" java.lang.IllegalArgumentException: Cannot reflectively create enum objects
at java.lang.reflect.Constructor.newInstance(Constructor.java:511)
at day20150730.NotSingletion.main(NotSingletion.java:25)

A ,B两种方法都弄出来了不同的实例，第三个没有构造器，所以没办法弄出别的实例来