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I'd like to learn something about javaagents, but researching is not easy. Most of result refers to JADE. I know java agent can mean two things:
I've found similar question here, but unfortunately it also refers to version 1.
Do you know any articles, tutorials for beginners, sample project about javaagent in version 2? I've found one here, but I'm looking for more.
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In general, a java agent is just a specially crafted jar file. It utilizes the Instrumentation API that the JVM provides to alter existing byte-code that is loaded in a JVM. For an agent to work, we need to define two methods: premain – will statically load the agent using -javaagent parameter at JVM startup.
To create a successful javaagent we'll need four things: an agent class, some meta-information to tell the JVM what capabilities to give to our agent class, a way to make the JVM load the . jar with the agent before it starts minding the application's business and a coffee.
Java agents are a special type of class which, by using the Java Instrumentation API, can intercept applications running on the JVM, modifying their bytecode.
The second case talks about Java Instrumentation API - this link points to a Javadoc which is rather descriptive.
And here, is the full instruction and an example of how to create java instrumentation agent.
The main concept is to:
Implement a static premain (as an analogy to main) method, like this:
import java.lang.instrument.Instrumentation; class Example { public static void premain(String args, Instrumentation inst) { ... } } Create a manifest file (say, manifest.txt) marking this class for pre-main execution. Its contents are:
Premain-Class: Example Compile the class and package this class into a JAR archive:
javac Example.java jar cmf manifest.txt yourAwesomeAgent.jar *.class Execute your JVM with -javaagent parameter, like this:
java -javaagent:yourAwesomeAgent.jar -jar yourApp.jar If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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