code manipulation : 바이트 코드 변경하기(javaagent)

code-manipulation/JavaAgent/pom.xml

@@ -0,0 +1,53 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <groupId>org.example</groupId>
+  <artifactId>JavaAgent</artifactId>
+  <version>1.0-SNAPSHOT</version>
+
+  <name>JavaAgent</name>
+
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    <maven.compiler.source>11</maven.compiler.source>
+    <maven.compiler.target>11</maven.compiler.target>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>net.bytebuddy</groupId>
+      <artifactId>byte-buddy</artifactId>
+      <version>1.10.20</version>
+    </dependency>
+  </dependencies>
+  <build>
+    <pluginManagement>
+      <plugins>
+        <plugin>
+          <groupId>org.apache.maven.plugins</groupId>
+          <artifactId>maven-jar-plugin</artifactId>
+          <version>3.1.2</version>
+          <configuration>
+            <archive>
+              <index>true</index>
+              <manifest>
+                <addClasspath>true</addClasspath>
+              </manifest>
+              <manifestEntries>
+                <mode>development</mode>
+                <url>${project.url}</url>
+                <key>value</key>
+                <Premain-Class>org.example.MasulsaAgent</Premain-Class>
+                <Can-Redefine-Classes>true</Can-Redefine-Classes>
+                <Can-Retransform-Classes>true</Can-Retransform-Classes>
+              </manifestEntries>
+            </archive>
+          </configuration>
+        </plugin>
+      </plugins>
+    </pluginManagement>
+  </build>
+</project>

code-manipulation/JavaAgent/src/main/java/org/example/MasulsaAgent.java

@@ -0,0 +1,16 @@
+package org.example;
+
+import net.bytebuddy.agent.builder.AgentBuilder;
+import net.bytebuddy.implementation.FixedValue;
+import net.bytebuddy.matcher.ElementMatchers;
+
+import java.lang.instrument.Instrumentation;
+
+public class MasulsaAgent {
+
+    public static void premain(String agentArgs, Instrumentation inst) {
+        new AgentBuilder.Default()
+                .type(ElementMatchers.any())
+                .transform((builder, typeDescription, classLoader, javaModule) -> builder.method(ElementMatchers.named("pullOut")).intercept(FixedValue.value("Rabbit!!"))).installOn(inst);
+    }
+}

code-manipulation/src/main/java/org/example/Masulsa.java

@@ -1,8 +1,10 @@
 package org.example;
 
 import net.bytebuddy.ByteBuddy;
+import net.bytebuddy.dynamic.ClassFileLocator;
 import net.bytebuddy.implementation.FixedValue;
 import net.bytebuddy.matcher.ElementMatchers;
+import net.bytebuddy.pool.TypePool;
 
 import java.io.File;
 import java.io.IOException;
@@ -11,20 +13,29 @@ public class Masulsa {
 
     public static void main(String[] args) {
 
+        System.out.println(new Moja().pullOut());
+
+
+
+        /* ============================================================= */
+
+//        ClassLoader classLoader = Masulsa.class.getClassLoader();
+//        TypePool typePool = TypePool.Default.of(classLoader);
+//
 //        /**
 //         * redefine() : Moja 클래스를 재정의
 //         * method() : Moja 클래스의 pullOut 메소드의 값을 Rabbit 으로 재정의
 //         * make().saveIn() : 재정의 후 지정한 위치에 저장 ( Moja.class 가 생성되는 파일 위치 지정 )
 //         */
 //        try {
-//            new ByteBuddy().redefine(Moja.class)
+//            new ByteBuddy().redefine(typePool.describe("org.example.Moja").resolve(),
+//                    ClassFileLocator.ForClassLoader.of(classLoader))
 //                    .method(ElementMatchers.named("pullOut")).intercept(FixedValue.value("Rabbit!!"))
 //                    .make().saveIn(new File("C:\\Users\\Woojin\\Desktop\\study\\Study\\code-manipulation\\target\\classes\\"));
 //        } catch (IOException e) {
 //            e.printStackTrace();
 //        }
-
+//
-        // 같이 실행하면 new Moja()에서 먼저 클래스가 로딩이 되기 때문에 위의 코드 재정의가 되지 않는다.
+//        System.out.println(new Moja().pullOut());
-        System.out.println(new Moja().pullOut());
     }
 }