This article explains how Java instrumentation works, and how you can write your own agent to do some basic profiling/tracing.
There is 2 ways to load the agent:
After the JVM has initialized, the agent class will be loaded by the system class loader. If the class loader fails to load the agent, the JVM will abort.
Next, the JVM instantiates an Instrumentation interface implementation and given the context, tries to invoke one of the two methods that an agent must implement: premain or agentmain.
The premain method is invoked after JVM initialization and the agentmain method is invoked sometime after the JVM has started (if the JVM provides such a mechanism). When the agent is started using a command-line option (with
The signatures of the premain method are:
And the signatures of the agentmain method are:
Now, all future class definitions will be seen by the transformer, except definitions of classes upon which any registered transformer is dependent. The transformer is called when classes are loaded, when they are redefined. and optionally, when they are retransformed (if the transformer was added to the instrumentation instance with the boolean canRetransform set to true).
The following method of the ClassFileTransformer interface is responsible of any class file transformation:
This is where things get complicated because you need to manipulate raw byte code. To achieve this, i strongly suggest to rely on dedicated tools such as ASM or Javassist.
The class to be instrumented is:
The agent class is:
Then, running the test should produce something like this:
Overview
JVM instrumentation feature was introduced in JDK 1.5 and is based on byte code instrumentation (BCI). Actually, when a class is loaded, you can alter the corresponding byte code to introduce features such as methods execution profiling or event tracing. Most of Java Application Performance Management (APM) solutions use this mechanism to monitor JVM.Instrumentation Agent
To enable JVM instrumentation, you have to provide an agent (or more) that is deployed as a JAR file. An attribute in the JAR file manifest specifies the agent class which will be loaded to start the agent.There is 2 ways to load the agent:
- with a command-line interface: by adding this option to the command-line:
-javaagent:jarpath[=options] where jarpath is the path to the agent JAR file. options is the agent options. This switch may be used multiple times on the same command-line, thus creating multiple agents. More than one agent may use the same jarpath. - by dynamic loading: the JVM must implement a mechanism to start agents sometime after the the VM has started. That way, a tool can "attach" an agent to a running JVM (for instance profilers or ByteMan)
After the JVM has initialized, the agent class will be loaded by the system class loader. If the class loader fails to load the agent, the JVM will abort.
Next, the JVM instantiates an Instrumentation interface implementation and given the context, tries to invoke one of the two methods that an agent must implement: premain or agentmain.
The premain method is invoked after JVM initialization and the agentmain method is invoked sometime after the JVM has started (if the JVM provides such a mechanism). When the agent is started using a command-line option (with
-javaagent), the agentmain method is not invoked. The agent class may also have a premain method for use when the agent is started using a command-line option. When the agent is started after JVM startup the premain method is not invoked.The signatures of the premain method are:
public static void premain(String agentArgs, Instrumentation inst); public static void premain(String agentArgs);
And the signatures of the agentmain method are:
public static void agentmain(String agentArgs, Instrumentation inst); public static void agentmain(String agentArgs);The agent needs to implement only one signature per method. The JVM first attempts to invoke the first signature, and if the agent class does not implement it then the JVM will attempt to invoke the second signature.
Byte Code Instrumentation
With the premain and agentmain methods, the agent can register a ClassFileTransformer instance by providing an implementation of this interface in order to transform class files. To register the transformer, the agent can use the addTransformer method of the given Instrumentation instance.
Now, all future class definitions will be seen by the transformer, except definitions of classes upon which any registered transformer is dependent. The transformer is called when classes are loaded, when they are redefined. and optionally, when they are retransformed (if the transformer was added to the instrumentation instance with the boolean canRetransform set to true).
The following method of the ClassFileTransformer interface is responsible of any class file transformation:
byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer);
Basic Profiling
To illustrate how the class file transformer can be used, we are going to set up some basic method profiling with Javassist:
- Write a trivial class that will be instrumented
- Write a ClassFileTransformer to inject some code to print method execution time
- Write an agent that registers the previous transformer
- Write the corresponding JUnit test
package org.javabenchmark.instrumentation;
public class Sleeping {
public void randomSleep() throws InterruptedException {
// randomly sleeps between 500ms and 1200s
long randomSleepDuration = (long) (500 + Math.random() * 700);
System.out.printf("Sleeping for %d ms ..\n", randomSleepDuration);
Thread.sleep(randomSleepDuration);
}
}
The transformer class is:
The transformer checks if the class to transform is the Sleeping one, then it injects the code that prints the time elapsed by the execution of the randomSleep() method.package org.javabenchmark.instrumentation;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
public class SleepingClassFileTransformer implements ClassFileTransformer {
public byte[] transform(ClassLoader loader, String className, Class classBeingRedefined,
ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
byte[] byteCode = classfileBuffer;
if (className.equals("org/javabenchmark/instrumentation/Sleeping")) {
try {
ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.get("org.javabenchmark.instrumentation.Sleeping");
CtMethod m = cc.getDeclaredMethod("randomSleep");
m.addLocalVariable("elapsedTime", CtClass.longType);
m.insertBefore("elapsedTime = System.currentTimeMillis();");
m.insertAfter("{elapsedTime = System.currentTimeMillis() - elapsedTime;"
+ "System.out.println(\"Method Executed in ms: \" + elapsedTime);}");
byteCode = cc.toBytecode();
cc.detach();
} catch (Exception ex) {
ex.printStackTrace();
}
}
return byteCode;
}
}
The agent class is:
package org.javabenchmark.instrumentation;
import java.lang.instrument.Instrumentation;
public class Agent {
public static void premain(String agentArgs, Instrumentation inst) {
// registers the transformer
inst.addTransformer(new SleepingClassFileTransformer());
}
}
And finally, the corresponding JUnit code:
package org.javabenchmark.instrumentation;
import org.junit.Test;
public class AgentTest {
@Test
public void shouldInstantiateSleepingInstance() throws InterruptedException {
Sleeping sleeping = new Sleeping();
sleeping.randomSleep();
}
}
But, before executing the test you need to setup Maven to enable the JVM agent:- build the JAR that contains the agent's code before running test
- add the JVM option -javagent to the JVM that runs the test
- add the Javassist dependency
org.apache.maven.plugins
maven-jar-plugin
2.4
process-classes
jar
org.javabenchmark.instrumentation.Agent
org.apache.maven.plugins
maven-surefire-plugin
2.14
-javaagent:target/${project.build.finalName}.jar
Next, add the Javassist dependency:
org.javassist javassist 3.14.0-GA jar
Then, running the test should produce something like this:
------------------------------------------------------- T E S T S ------------------------------------------------------- Running org.javabenchmark.instrumentation.AgentTest Sleeping for 818 ms .. Method Executed in ms: 820 Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 1.001 secYou can see that there is a new trace: Method Executed in ms: 820 proving that instrumentation works. Without instrumentation, you would only have the Sleeping for 818 ms .. trace.
Conclusion
It is easy to profile your code with the instrumentation API and the Javassist API: write transformers with Javassist, write an agent to register them, then use the -javaagent option and you're done !
