UP

isXxxSupported()

public interface Instrumentation {
    boolean isRedefineClassesSupported();
    boolean isRetransformClassesSupported();
    boolean isNativeMethodPrefixSupported();
}
  • boolean isRedefineClassesSupported(): Returns whether or not the current JVM configuration supports redefinition of classes.
    • The ability to redefine an already loaded class is an optional capability of a JVM.
    • Redefinition will only be supported if the Can-Redefine-Classes manifest attribute is set to true in the agent JAR file and the JVM supports this capability.
    • During a single instantiation of a single JVM, multiple calls to this method will always return the same answer.
  • boolean isRetransformClassesSupported(): Returns whether or not the current JVM configuration supports retransformation of classes.
    • The ability to retransform an already loaded class is an optional capability of a JVM.
    • Retransformation will only be supported if the Can-Retransform-Classes manifest attribute is set to true in the agent JAR file and the JVM supports this capability.
    • During a single instantiation of a single JVM, multiple calls to this method will always return the same answer.
  • boolean isNativeMethodPrefixSupported(): Returns whether the current JVM configuration supports setting a native method prefix.
    • The ability to set a native method prefix is an optional capability of a JVM.
    • Setting a native method prefix will only be supported if the Can-Set-Native-Method-Prefix manifest attribute is set to true in the agent JAR file and the JVM supports this capability.
    • During a single instantiation of a single JVM, multiple calls to this method will always return the same answer.

小总结：

  • 第一，判断 JVM 是否支持该功能。
  • 第二，判断 Java Agent Jar 内的 MANIFEST.MF 文件里的属性是否为 true。
  • 第三，在一个 JVM 实例当中，多次调用某个 isXxxSupported() 方法，该方法的返回值是不会改变的。

示例

LoadTimeAgent.java

package lsieun.agent;

import java.lang.instrument.Instrumentation;

public class LoadTimeAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        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());
    }
}

运行

在 pom.xml 文件中，maven-jar-plugin 处可以设置 Can-Redefine-Classes、Can-Retransform-Classes 和 Can-Set-Native-Method-Prefix 属性。

第一次测试时，将三个属性设置为 true：

<manifestEntries>
    <Premain-Class>lsieun.agent.LoadTimeAgent</Premain-Class>
    <Agent-Class>lsieun.agent.DynamicAgent</Agent-Class>
    <Can-Redefine-Classes>true</Can-Redefine-Classes>
    <Can-Retransform-Classes>true</Can-Retransform-Classes>
    <Can-Set-Native-Method-Prefix>true</Can-Set-Native-Method-Prefix>
</manifestEntries>

第二次测试时，将三个属性设置为 false：

<manifestEntries>
    <Premain-Class>lsieun.agent.LoadTimeAgent</Premain-Class>
    <Agent-Class>lsieun.agent.DynamicAgent</Agent-Class>
    <Can-Redefine-Classes>false</Can-Redefine-Classes>
    <Can-Retransform-Classes>false</Can-Retransform-Classes>
    <Can-Set-Native-Method-Prefix>false</Can-Set-Native-Method-Prefix>
</manifestEntries>

每次测试之前，都需要重新生成 .jar 文件：

mvn clean package

第一次运行，将三个属性设置成 true，示例输出：

$ 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

第二次运行，将三个属性设置成 false，示例输出：

$ java -cp ./target/classes/ -javaagent:./target/TheAgent.jar sample.Program
Premain-Class: lsieun.agent.LoadTimeAgent
Can-Redefine-Classes: false
Can-Retransform-Classes: false
Can-Set-Native-Method-Prefix: false

总结

本文内容总结如下：

  • 第一点，判断某一个 isXxxSupported() 方法是否为 true，需要考虑两个因素：
    • 判断 JVM 是否支持该功能。
    • 判断 Agent.jar 内的 MANIFEST.MF 文件里的属性是否为 true。
  • 第二点，在一个 JVM 实例当中，多次调用某个 isXxxSupported() 方法，该方法的返回值是不会改变的。