Med Java

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====== Dynamic Classpath Loading with Java Instrumentation ======

===== Overview =====
Starting with Java 9, the recommended way to modify the classpath at runtime is using Java Instrumentation. This approach works with the Java Module System and is production-ready.

===== Implementation =====

==== Basic Agent Class ====
<code java>
package com.example;

import java.lang.instrument.Instrumentation;
import java.util.jar.JarFile;

public class DynamicLoader {
    private static Instrumentation instrumentation;
    
    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
        try {
            // Load specific jar
            inst.appendToSystemClassLoaderSearch(new JarFile("libs/mylib.jar"));
            
            // Or load all jars from a directory
            File libDir = new File("libs");
            if (libDir.isDirectory()) {
                for (File jar : libDir.listFiles((dir, name) -> name.endsWith(".jar"))) {
                    inst.appendToSystemClassLoaderSearch(new JarFile(jar));
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}
</code>

==== MANIFEST.MF ====
<code>
Manifest-Version: 1.0
Premain-Class: com.example.DynamicLoader
Can-Redefine-Classes: true
Can-Retransform-Classes: true
</code>

===== Building =====

==== Without Maven ====
<code bash>
javac DynamicLoader.java

mkdir META-INF
# Create MANIFEST.MF with content above
jar cfm dynamic-loader.jar META-INF/MANIFEST.MF com/example/DynamicLoader.class
</code>

==== With Maven ====
<code xml>
<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>dynamic-loader</artifactId>
    <version>1.0-SNAPSHOT</version>
    
    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
    </properties>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-jar-plugin</artifactId>
                <version>3.2.0</version>
                <configuration>
                    <archive>
                        <manifest>
                            <addClasspath>true</addClasspath>
                        </manifest>
                        <manifestEntries>
                            <Premain-Class>com.example.DynamicLoader</Premain-Class>
                            <Can-Redefine-Classes>true</Can-Redefine-Classes>
                            <Can-Retransform-Classes>true</Can-Retransform-Classes>
                        </manifestEntries>
                    </archive>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>
</code>

===== Running =====

==== Basic Usage ====
<code bash>
java -javaagent:dynamic-loader.jar --add-opens java.base/java.lang=ALL-UNNAMED YourMainClass
</code>

==== Example Main Class ====
<code java>
public class MainApp {
    public static void main(String[] args) {
        // Classes from dynamically loaded jars are now available
        MyDynamicClass instance = new MyDynamicClass();
        instance.doSomething();
    }
}
</code>

===== Important Notes =====
  * The agent must be loaded before the main class
  * The --add-opens argument is required for Java 9+
  * The agent jar must be accessible when starting the JVM
  * Error handling should be implemented for production use

===== Common Issues =====
  * **ClassNotFoundException**: Verify jar path and manifest
  * **SecurityException**: Check --add-opens arguments
  * **InvalidJarException**: Ensure jar files are valid

===== Best Practices =====
  * Log all loaded jars
  * Implement proper error handling
  * Document the loading process
  * Test with all target Java versions
  * Consider security implications

===== Example Directory Structure =====
<code>
project/
│
├── src/
│   └── main/
│       └── java/
│           └── com/
│               └── example/
│                   └── DynamicLoader.java
│
├── libs/
│   ├── mylib1.jar
│   └── mylib2.jar
│
├── pom.xml
└── README.md
</code>

===== References =====
  * [[https://docs.oracle.com/en/java/javase/17/docs/api/java.instrument/java/lang/instrument/Instrumentation.html|Java Instrumentation API]]
  * [[https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/util/jar/JarFile.html|JarFile Documentation]]