redefineModule

public interface Instrumentation {
    boolean isModifiableModule(Module module);
    void redefineModule (Module module,
                         Set<Module> extraReads,
                         Map<String, Set<Module>> extraExports,
                         Map<String, Set<Module>> extraOpens,
                         Set<Class<?>> extraUses,
                         Map<Class<?>, List<Class<?>>> extraProvides);
}
  • isModifiableModule: Tests whether a module can be modified with redefineModule.
  • redefineModule: Redefine a module to expand the set of modules that it reads, the set of packages that it exports or opens, or the services that it uses or provides.

示例

Application

package sample;

import java.lang.instrument.Instrumentation;

public class Program {
    public static void main(String[] args) {
        Module baseModule = Object.class.getModule();
        Module instrumentModule = Instrumentation.class.getModule();

        boolean canRead = baseModule.canRead(instrumentModule);
        String message = String.format("%s can read %s: %s", baseModule.getName(), instrumentModule.getName(), canRead);
        System.out.println(message);
    }
}

Agent Jar

package lsieun.agent;

import java.lang.instrument.Instrumentation;
import java.util.Map;
import java.util.Set;

public class LoadTimeAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        // 第一步，打印信息
        System.out.println("Premain-Class: " + LoadTimeAgent.class.getName());
        System.out.println("Can-Redefine-Classes: " + inst.isRedefineClassesSupported());
        System.out.println("Can-Retransform-Classes: " + inst.isRetransformClassesSupported());
        System.out.println("Can-Set-Native-Method-Prefix: " + inst.isNativeMethodPrefixSupported());
        System.out.println("========= ========= =========");

        // 第二步，判断一个 module 是否可以读取另一个 module
        Module baseModule = Object.class.getModule();
        Module instrumentModule = Instrumentation.class.getModule();
        boolean canRead = baseModule.canRead(instrumentModule);

        // 第三步，使用 inst：修改 module 权限
        if (!canRead && inst.isModifiableModule(baseModule)) {
            Set<Module> extraReads = Set.of(instrumentModule);
            inst.redefineModule(baseModule, extraReads, Map.of(), Map.of(), Set.of(), Map.of());
        }
    }
}

Run

None

$ java -cp ./target/classes/ sample.Program
java.base can read java.instrument: false

Load-Time

$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program
Premain-Class: lsieun.agent.LoadTimeAgent
Can-Redefine-Classes: true
Can-Retransform-Classes: true
Can-Set-Native-Method-Prefix: true
========= ========= =========
java.base can read java.instrument: true

总结

本文内容总结如下：

  • 第一点， redefineModule 方法的作用是对 module 的访问权限进行修改，该方法是在 Java 9 引入的。