java-Agent-demo

public class HelloWorld {
    public static void main(String arg[]) {
        System.out.println("The age is 9");
    }
}

import java.lang.instrument.Instrumentation;

public class MySimpleAgent {
    public static void premain(String agentArgs,Instrumentation inst) {
        inst.addTransformer(new MySimpleTransformer());
    }
}

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;

public class MySimpleTransformer implements ClassFileTransformer {
    public byte[] transform(ClassLoader classloader,
                            String classname,
                            Class redefinedclass,
                            ProtectionDomain protectiondomain,
                            byte b[]) throws IllegalClassFormatException {
        if(!classname.endsWith("HelloWorld"))
            return(null);

        String line = "";
        for(int i=0; i < b.length;i++){
            line += Byte.toString(b[i]) + " ";
            if(line.length() > 60) {
                System.out.println(line);
                line = "";
            }
            if(b[i] == (byte)'9')
                b[i] = (byte)'0';
        }
        System.out.println(line);
        System.out.println("The number of bytes in HelloWorld: " + b.length);
        return(b);
    }
}

MANIFEST.MF :(此处有坑，:号后要有空格，否则打jar包回报错！)

Manifest-Version: 1.0
Premain-Class: MySimpleAgent
Created-By: 1.8.0_151 (Oracle Corporation)

打包报错：
java.io.IOException: invalid header field
    at java.util.jar.Attributes.read(Attributes.java:410)
    at java.util.jar.Manifest.read(Manifest.java:199)
    at java.util.jar.Manifest.<init>(Manifest.java:69)
    at sun.tools.jar.Main.run(Main.java:176)
    at sun.tools.jar.Main.main(Main.java:1288)

插播一条命令：删除.class 结尾：

find . -name "*.class" | xargs rm

打包命令：

javac *.java

打Agent jar包

jar cvfm MySimpleAgent.jar MANIFEST.MF MySimpleAgent.class MySimpleTransformer.class

运行HelloWorld

java -javaagent:MySimpleAgent.jar HelloWorld

运行结果：

 ~/dev/my-process/my-dev-doc/agent  java -javaagent:MySimpleAgent.jar HelloWorld
objc[52894]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/bin/java (0x10eb484c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_151.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x10ebd44e0). One of the two will be used. Which one is undefined.
-54 -2 -70 -66 0 0 0 52 0 29 10 0 6 0 15 9 0 16 0 17 8 0 18 10
0 19 0 20 7 0 21 7 0 22 1 0 6 60 105 110 105 116 62 1 0 3 40
41 86 1 0 4 67 111 100 101 1 0 15 76 105 110 101 78 117 109 98
101 114 84 97 98 108 101 1 0 4 109 97 105 110 1 0 22 40 91 76
106 97 118 97 47 108 97 110 103 47 83 116 114 105 110 103 59
41 86 1 0 10 83 111 117 114 99 101 70 105 108 101 1 0 15 72 101
108 108 111 87 111 114 108 100 46 106 97 118 97 12 0 7 0 8 7
0 23 12 0 24 0 25 1 0 12 84 104 101 32 97 103 101 32 105 115
32 57 7 0 26 12 0 27 0 28 1 0 10 72 101 108 108 111 87 111 114
108 100 1 0 16 106 97 118 97 47 108 97 110 103 47 79 98 106 101
99 116 1 0 16 106 97 118 97 47 108 97 110 103 47 83 121 115 116
101 109 1 0 3 111 117 116 1 0 21 76 106 97 118 97 47 105 111
47 80 114 105 110 116 83 116 114 101 97 109 59 1 0 19 106 97
118 97 47 105 111 47 80 114 105 110 116 83 116 114 101 97 109
1 0 7 112 114 105 110 116 108 110 1 0 21 40 76 106 97 118 97
47 108 97 110 103 47 83 116 114 105 110 103 59 41 86 0 33 0 5
0 6 0 0 0 0 0 2 0 1 0 7 0 8 0 1 0 9 0 0 0 29 0 1 0 1 0 0 0 5
42 -73 0 1 -79 0 0 0 1 0 10 0 0 0 6 0 1 0 0 0 1 0 9 0 11 0 12
0 1 0 9 0 0 0 37 0 2 0 1 0 0 0 9 -78 0 2 18 3 -74 0 4 -79 0 0
0 1 0 10 0 0 0 10 0 2 0 0 0 3 0 8 0 4 0 1 0 13 0 0 0 2 0 14
The number of bytes in HelloWorld: 426
The age is 0
 ~/dev/my-process/my-dev-doc/agent 

age 由 9 更改成 0 ！

所有文件如图：

image.png