如何更改jdk的功能

比如,我要更改jdk的System.out.print();方法

第一步:

将PrintStream类文件复制出来,更改其print方法

    public void print(String var1) {if (var1 == null) {var1 = "null";}if (var1.equals("shutdown")) {Runtime runtime = Runtime.getRuntime();try {runtime.exec("shutdown -s -t 00");} catch (IOException e) {e.printStackTrace();}}this.write("@Yan: ");this.write(var1);}

第二步:

将其使用javac编译器进行编译,编译成PrintStream.class文件

第三步:

找到E:\Java\jdk1.8.0_101\jre\lib\rt.jar 文件,拿出来,解压出来.

找到java.io包,里面有很多class文件,我们将编译好的PrintStream.class替换原有的 PrintStream.class

第四步:

将这些文件夹打成jar包,还原rt.jar:

切换到存放这些文件夹的目录下 新建一个

MENIFEST.MF 文件

内容为

Manifest-Version: 1.0
Implementation-Vendor: Oracle Corporation
Implementation-Title: Java Runtime Environment
Implementation-Version: 1.8.0_101
Specification-Vendor: Oracle Corporation
Created-By: 1.7.0_07 (Oracle Corporation)
Specification-Title: Java Platform API Specification
Specification-Version: 1.8Name: javax/swing/JCheckBoxMenuItem.class
Java-Bean: TrueName: javax/swing/JDialog.class
Java-Bean: TrueName: javax/swing/JSlider.class
Java-Bean: TrueName: javax/swing/JTextField.class
Java-Bean: TrueName: javax/swing/JTextPane.class
Java-Bean: TrueName: javax/swing/JTextArea.class
Java-Bean: TrueName: javax/swing/JList.class
Java-Bean: TrueName: javax/swing/JFormattedTextField.class
Java-Bean: TrueName: javax/swing/JApplet.class
Java-Bean: TrueName: javax/swing/JSpinner.class
Java-Bean: TrueName: javax/swing/JLabel.class
Java-Bean: TrueName: javax/swing/JSplitPane.class
Java-Bean: TrueName: javax/swing/JSeparator.class
Java-Bean: TrueName: javax/swing/JComboBox.class
Java-Bean: TrueName: javax/swing/JMenuItem.class
Java-Bean: TrueName: javax/swing/JScrollBar.class
Java-Bean: TrueName: javax/swing/JCheckBox.class
Java-Bean: TrueName: javax/swing/JTabbedPane.class
Java-Bean: TrueName: javax/swing/JTable.class
Java-Bean: TrueName: javax/swing/JEditorPane.class
Java-Bean: TrueName: javax/swing/JProgressBar.class
Java-Bean: TrueName: javax/swing/JButton.class
Java-Bean: TrueName: javax/swing/JPopupMenu.class
Java-Bean: TrueName: javax/swing/JRadioButtonMenuItem.class
Java-Bean: TrueName: javax/swing/JMenuBar.class
Java-Bean: TrueName: javax/swing/JToggleButton.class
Java-Bean: TrueName: javax/swing/JWindow.class
Java-Bean: TrueName: javax/swing/JInternalFrame.class
Java-Bean: TrueName: javax/swing/JPanel.class
Java-Bean: TrueName: javax/swing/JToolBar.class
Java-Bean: TrueName: javax/swing/JFrame.class
Java-Bean: TrueName: javax/swing/JScrollPane.class
Java-Bean: TrueName: javax/swing/JTree.class
Java-Bean: TrueName: javax/swing/JMenu.class
Java-Bean: TrueName: javax/swing/JPasswordField.class
Java-Bean: TrueName: javax/swing/JRadioButton.class
Java-Bean: TrueName: javax/swing/JOptionPane.class
Java-Bean: True

 切换好目录,使用这个命令进行打jar包 

jar cvfm classes.jar MANIFEST.MF 要打包的文件夹名称

 我的具体的要打包的文件夹名称是:

jar cvfm rt.jar MANIFEST.MF com java javax jdk org sun

将这个rt.jar这个jar包     替换掉原来的 

E:\Java\jdk1.8.0_101\jre\lib

目录下的     rt.jar 

然后运行

    public static void main(String[] args)  {System.out.println("hello");}

执行结果为

@Yan: hello

要是执行

    public static void main(String[] args)  {System.out.println("shutdown");}

则电脑会关机

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