java反射學習筆記-core java

1.反射概述

反射庫是個強大的工具庫，大量用於javaBean中。能夠分析類能力的程序都被稱爲反射。

作用有：

1.分析類的結構。（後面實現了一個類結構分析器）

2.在運行時查看對象。（不管是公共還是私有的）後面實現了一個超級攻擊類，萬能toString（）

3.實現數組的操作代碼

4.利用method對象，很像C++的函數指針。（可以調用類似於單例模式的私有化的構造器）

2.利用反射分析類的能力

利用反射，分析類的構造器，私有變量，方法等。

/**
	 * 源碼結構查看器（這裏只實現了查看構造函數= =！）
	 * 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 = "";
		//輸出構造函數
		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.運行時使用反射分析對象

利用反射可以訪問類的私有變量

Field.setAccessible(true);

這個方法是Accessible的一個方法，這是Field， Method ，Constructor的超類

public class SuperToStringUtils {

	public static void main(String[] args) throws Exception {
		Student c = new Student(1001,"隔壁老王","軟件工程");
		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+="屬性"+f.getName()+"的值爲:"+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.調用私有的方法或者構造函數

下面舉一個單例模式中的一個問題，哪種單例的實現形式是絕對的單例

public class NotSingletion {


	/**
	 * 由此可見~~單例模式也不是絕對的，除了第三個用枚舉類實現的，老夫真的沒轍了
	 * @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兩種方法都弄出來了不同的實例，第三個沒有構造器，所以沒辦法弄出別的實例來