对Java反射机制的认识

1.        Java反射机制是动态语言的一种特质

动态类型语言和静态类型语言的主要区别：在编译期检查变量类型还是在运行期检查编译类型，比如静态语言：C,C++ 动态语言：Shell，python，JS 等。而反射允许Java把对某些类的类型检查的过程推迟到运行时。

 

2.        反射的特点

反射机制允许程序在运行时透过Reflection APIs取得任何一个已知名称的class的内部信息，允许运行中的 Java 程序对自身进行检查，并能直接操作程序的内部属性，甚至创建某个类的实例。这种编程方式可以让对象在生成时才决定要生成哪一种对象。

 

3.        反射的优缺点

反射提高了编程时的灵活性，但是对性能有影响，就像HashMap和HashTable一样。

 

4.        Java反射的核心类

java.lang.Class、java.lang.reflect.*Field[] filedsList =( Class.forName(className)).getFields();Method[] methodsList = =( Class.forName(className)).getMethods();Constructor cons = testClass.getConstructor(argsClass);Object o = cons.newInstance(args);

例子：

import java.lang.reflect.*;public class TestReflect {public static void printFields(Field[] fields ){for( int i=0; i<fields.length; i++)System.out.println( fields[i].toString());}public static void printMethods(Method[] methods ){for( int i=0; i<methods.length; i++)System.out.println( methods[i].toString());}//运行时获取某个类的属性名和方法名public void getProptiesAndMethods(String className){try {Class testClass = Class.forName(className);Field[] filedsList = testClass.getFields();//System.out.println("各个字段:");printFields(filedsList);Method[] methodsList = testClass.getMethods();//System.out.println("\n各个方法:");printMethods(methodsList);} catch (ClassNotFoundException e) {e.printStackTrace();}}//运行时新建某个类的实例public Object getNewInstance(String className, Object[] args){Object o = null;try {Class testClass = Class.forName(className);Class[] argsClass = new Class[args.length];                                  for (int i = 0, j = args.length; i < j; i++) {                                    argsClass[i] = args[i].getClass();                                       }   Constructor cons = testClass.getConstructor(argsClass); o = cons.newInstance(args);} catch (Exception e) {e.printStackTrace();}return o;}public int incrementField(String name, Object obj) {int value = 0;try{    Field field = obj.getClass().getDeclaredField(name);    value = field.getInt(obj) + 1;    field.setInt(obj, value);} catch (Exception e){e.printStackTrace();}    return value;}public static void main(String[] args){String classname = "java.lang.String";Test t = new Test("TT", 22);TestReflect tr = new TestReflect();tr.getProptiesAndMethods("java.util.Scanner");//Object[] cargs = {"TT"};//System.out.println(//((String)tr.getNewInstance("java.lang.String", cargs)).charAt(0) );////t.introd();//tr.incrementField("age", t);//t.introd();}

5.        用Java反射实现一个类的探查工具

1百多行，就贴下源码：

package hq.net.csdn.blog;import java.awt.event.ActionEvent;import java.awt.event.ActionListener;import java.lang.reflect.Field;import java.lang.reflect.Method;import javax.swing.JFrame;import javax.swing.JLabel;import javax.swing.JOptionPane;import javax.swing.JScrollPane;import javax.swing.JTextField;import javax.swing.JButton;import javax.swing.JTextArea;/** * get class info by the class name. */public class GetClassInfo extends JFrame{private static final long serialVersionUID = 1L;private static GetClassInfo mainWindow = null;private final int FRAME_WIDTH = 700;private final int FRAME_HEIGHT = 600;private final int PADDING_LEFT = 40;private final int BUTTON_HEIGTH = 30;private final int BUTTON_WIDTH = 50;private JLabel oLabel = null;private JTextField oTextField = null;private JButton oCheckButton = null;private JTextArea oTextArea = null;private JScrollPane oScrollPane = null;public GetClassInfo(){super();this.setSize(FRAME_WIDTH, FRAME_HEIGHT);this.getContentPane().setLayout(null);this.add(getJLabel(), null);this.add(getJTextField(), null);this.add(getCheckButton(), null);this.add(getTextArea(), null);this.add(getScrollPane(), null);this.setTitle("Reference Class");this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);}//labelprivate JLabel getJLabel() {if ( null == oLabel ) {oLabel = new JLabel();oLabel.setBounds(PADDING_LEFT, 20, 150, BUTTON_HEIGTH);oLabel.setText("请输入ClassName:");}return oLabel;}//text edit field for inputing class name private JTextField getJTextField() {if ( null == oTextField ) {oTextField = new JTextField();oTextField.setBounds(PADDING_LEFT+130, 20, 330, BUTTON_HEIGTH);}return oTextField;}//check buttonprivate JButton getCheckButton(){if( null == oCheckButton ){oCheckButton = new JButton();oCheckButton.setBounds(PADDING_LEFT+500, 20, BUTTON_WIDTH*2, BUTTON_HEIGTH);oCheckButton.setText("Check");oCheckButton.addActionListener( new ActionListener(){@Overridepublic void actionPerformed(ActionEvent evn) {String classname = oTextField.getText();oTextField.setText("");//clear edit text filedif( null==classname || "".equals( classname.trim())){JOptionPane.showMessageDialog(mainWindow, "Please Input Class Name!");return;}String classinfo = getClassInfo(classname);if( null==classinfo || "".equals( classinfo )) {JOptionPane.showMessageDialog(mainWindow, classname+ ": Class Not Found!");return;}oTextArea.setText( classinfo );}});}return oCheckButton;}//text area to display class info stringpublic JTextArea getTextArea(){if( null == oTextArea ){oTextArea = new JTextArea();}return oTextArea;}//scroll panelpublic JScrollPane getScrollPane(){if( null == oScrollPane ){oScrollPane = new JScrollPane(oTextArea);oScrollPane.setBounds(PADDING_LEFT, 60, FRAME_WIDTH-100, FRAME_HEIGHT-150);}return oScrollPane;}//the main!public static void main(String args[]){mainWindow = new GetClassInfo();mainWindow.setVisible(true);}public String getClassInfo(String classname ){Class cl = null;Method[] methods = null;Field[] fields = null;String newline = "\r\n";String classinfo = null;//try to get class info by the classnametry{classname = classname.trim();cl = Class.forName(classname);methods = cl.getDeclaredMethods();//get all methods and fields,fields = cl.getDeclaredFields();  //include private, protected and public } catch (Exception e){e.printStackTrace();return classinfo;}classinfo = "---"+classname +"---"+ newline + "Fields:" + newline; for( int i=0; i<fields.length; i++){classinfo += fields[i].toString() + newline;}classinfo += newline + "Methods:" + newline;for( int i=0; i<methods.length; i++){classinfo += methods[i].toString() + newline;}return classinfo;}}