writing-instrumentation-module.md

Write an InstrumentationModule step by step

The InstrumentationModule is the central piece of any OpenTelemetry javaagent instrumentation. There are many conventions that our javaagent uses, many pitfalls, and not so obvious patterns that one has to follow when implementing a module.

Here we describe how a javaagent instrumentation can be implemented and document the main aspects that may affect your instrumentation. In addition to this file, we suggest reading the InstrumentationModule and TypeInstrumentation Javadocs, as they often provide more detailed explanations of how to use a particular method and why it works the way it does.

The centerpiece of each javaagent instrumentation: InstrumentationModule

An InstrumentationModule describes a set of individual TypeInstrumentation that need to be applied together to correctly instrument a specific library. Type instrumentations grouped in a module share helper classes, muzzle runtime checks, and applicable class loader criteria, and can only be enabled or disabled as a set.

The OpenTelemetry javaagent finds all modules by using Java's ServiceLoader API. To make your instrumentation visible, make sure that a proper META-INF/services/ file is present in the javaagent jar. The easiest way to do it is using @AutoService:

@AutoService(InstrumentationModule.class)
public class MyLibraryInstrumentationModule extends InstrumentationModule {
  // ...
}

An InstrumentationModule needs to have at least one name. The user of the javaagent can suppress a chosen instrumentation by referring to it by one of its names. The instrumentation module names use kebab-case. The main instrumentation name, which is the first one, must be the same as the gradle module name, excluding the version suffix if present.

public MyLibraryInstrumentationModule() {
  super("my-library", "my-library-1.0");
}

For detailed information on InstrumentationModule names, see the InstrumentationModule#InstrumentationModule(String, String...) Javadoc.

Change the order of applying instrumentation modules using the order() method

To apply instrumentations in a specific order you can override the order() method to specify an order, like in the following snippet:

@Override
public int order() {
  return 1;
}

The higher the value returned by order() the later the instrumentation module is applied. Default value is 0.

Tell the agent which classes are a part of the instrumentation by overriding the isHelperClass() method

The OpenTelemetry javaagent picks up helper classes used in the instrumentation/advice classes and injects them into the application classpath. The agent can automatically find those classes that follow our package conventions, but it is also possible to explicitly tell which packages/classes are supposed to be treated as helper classes by implementing isHelperClass(String):

@Override
public boolean isHelperClass(String className) {
  return className.startsWith("org.my.library.opentelemetry");
}

For more information on package conventions, see the muzzle docs.

Inject additional resources using the registerHelperResources(HelperResourceBuilder) method

Some libraries may expose SPI interfaces that you can easily implement to provide telemetry-gathering capabilities. The OpenTelemetry javaagent is able to inject ServiceLoader service provider files, but it needs to be told which ones:

@Override
public void registerHelperResources(HelperResourceBuilder helperResourceBuilder) {
    helperResourceBuilder.register("META-INF/services/org.my.library.SpiClass");
}

All classes referenced by service providers defined in the helperResourceNames() method are treated as helper classes: they're checked for invalid references and automatically injected into the application class loader.

Inject additional instrumentation helper classes manually with the getAdditionalHelperClassNames() method

If you don't use the muzzle gradle plugins, or are in a scenario that requires providing the helper classes manually (for example, an unusual SPI implementation), you can override the getAdditionalHelperClassNames() method to provide a list of additional helper classes to be injected into the application class loader when the instrumentation is applied:

public List<String> getAdditionalHelperClassNames() {
  return Arrays.asList(
      "org.my.library.instrumentation.SomeHelper",
      "org.my.library.instrumentation.AnotherHelper");
}

The order of the class names returned by this method matters: if you have several helper classes extending one another, you'll want to return the base class first. For example, if you have a B extends A class, the list should contain A first and B second. The helper classes are injected into the application class loader after those provided by the muzzle codegen plugin.

Restrict the criteria for applying the instrumentation by extending the classLoaderMatcher() method

Different versions of the same library often need completely different instrumentations: for example, servlet 3 introduces several new async classes that need to be instrumented to produce correct telemetry data. An InstrumentationModule can define additional criteria for checking whether an instrumentation should be applied:

@Override
public ElementMatcher.Junction<ClassLoader> classLoaderMatcher() {
  return hasClassesNamed("org.my.library.Version2Class");
}

The above example skips instrumenting the application code if it does not contain the class introduced in the version covered by your instrumentation.

Define instrumented types with the typeInstrumentations() method

As last step, an InstrumentationModule implementation must provide at least one TypeInstrumentation implementation. A module with no type instrumentations does nothing.

@Override
public List<TypeInstrumentation> typeInstrumentations() {
  return Collections.singletonList(new MyTypeInstrumentation());
}

Describe the changes applied to a type using TypeInstrumentation class

A TypeInstrumentation describe the changes that need to be made to a single type. Depending on the instrumented library, they might only make sense in conjunction with other type instrumentations, grouped together in a module.

public class MyTypeInstrumentation implements TypeInstrumentation {
  // ...
}

Define which Java types should qualify for instrumentation by overriding the typeMatcher() method

A type instrumentation needs to declare what class (or classes) are going to be instrumented:

@Override
public ElementMatcher<TypeDescription> typeMatcher() {
  return named("org.my.library.SomeClass");
}

Make the agent faster by implementing classLoaderOptimization() method

When you need to instrument all classes that implement a particular interface, or all classes that are annotated with a particular annotation, implement the classLoaderOptimization() method. Matching classes by their name is quite fast, but inspecting the actual bytecode (for example, implements, has annotation, has method, etc.) is a rather expensive operation.

The matcher returned by the classLoaderOptimization() method makes the TypeInstrumentation significantly faster when instrumenting applications that do not contain the library:

@Override
public ElementMatcher<ClassLoader> classLoaderOptimization() {
  return hasClassesNamed("org.my.library.SomeInterface");
}

@Override
public ElementMatcher<? super TypeDescription> typeMatcher() {
  return implementsInterface(named("org.my.library.SomeInterface"));
}

Define the actual code transformations with the transform(TypeTransformer) method

This method describes what transformations should be applied to the matched type. The interface TypeTransformer, implemented internally by the agent, defines a set of available transformations that you can apply:

• applyAdviceToMethod(ElementMatcher<? super MethodDescription>, String) lets you apply an advice class (the second parameter) to all matching methods (the first parameter). We suggest to make the method matchers as strict as possible: the type instrumentation should only instrument the code that it targets.
• applyTransformer(AgentBuilder.Transformer) lets you to inject an arbitrary ByteBuddy transformer. This is an advanced, low-level option that is not subjected to muzzle safety checks and helper class detection. Use it responsibly.

Consider the following example:

@Override
public void transform(TypeTransformer transformer) {
  transformer.applyAdviceToMethod(
    isPublic()
        .and(named("someMethod"))
        .and(takesArguments(2))
        .and(takesArgument(0, String.class))
        .and(takesArgument(1, named("org.my.library.MyLibraryClass"))),
    this.getClass().getName() + "$MethodAdvice");
}

For matching built-in Java types you can use the takesArgument(0, String.class) form. Classes originating from the instrumented library need to be matched using the named() matcher.

Implementations of TypeInstrumentation often embed advice classes as static inner classes. These classes are referred to by name when applying advice classes to methods in the transform() method.

You might have noticed in the example above that the advice class is being referenced as follows:

this.getClass().getName() + "$MethodAdvice"

Referring to the inner class and calling getName() would be easier to read and understand, but note that this is intentional and should be maintained. Instrumentation modules are loaded by the agent's class loader, and this string concatenation is an optimization that prevents the actual advice class from being loaded into the agent's class loader.

Use advice classes to write code that will get injected to the instrumented library classes

Advice classes aren't really classes in that they're raw pieces of code that are pasted directly into the instrumented library class files. You should not treat them as ordinary, plain Java classes.

Unfortunately many standard practices do not apply to advice classes:

• If they're inner classes, they MUST be static.
• They MUST only contain static methods.
• They MUST NOT contain any state (fields) whatsoever, static constants included. Only the advice methods' content is copied to the instrumented code, constants are not.
• Inner advice classes defined in an InstrumentationModule or a TypeInstrumentation MUST NOT use anything from the outer class (loggers, constants, etc).
• Reusing code by extracting a common method and/or parent class won't work: create additional helper classes to store any reusable code instead.
• They SHOULD NOT contain any methods other than @Advice-annotated method.

Consider the following example:

@SuppressWarnings("unused")
public static class MethodAdvice {
  @Advice.OnMethodEnter(suppress = Throwable.class)
  public static void onEnter(/* ... */) {
    // ...
  }

  @Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)
  public static void onExit(/* ... */) {
    // ...
  }
}

Include the suppress = Throwable.class property in @Advice-annotated methods. Exceptions thrown by the advice methods get caught and handled by a special ExceptionHandler that the OpenTelemetry javaagent defines. The handler makes sure to properly log all unexpected exceptions.

The OnMethodEnter and OnMethodExit advice methods often need to share several pieces of information. We use local variables prefixed with otel to pass context, scope, and other data between both methods, like in the following example:

@Advice.OnMethodEnter(suppress = Throwable.class)
public static void onEnter(@Advice.Argument(1) Object request,
                           @Advice.Local("otelContext") Context context,
                           @Advice.Local("otelScope") Scope scope) {
  // ...
}

For telemetry-producing instrumentations, both methods follow this pattern:

@Advice.OnMethodEnter(suppress = Throwable.class)
public static void onEnter(@Advice.Argument(1) Object request,
                           @Advice.Local("otelContext") Context context,
                           @Advice.Local("otelScope") Scope scope) {
  Context parentContext = Java8BytecodeBridge.currentContext();

  if (!instrumenter().shouldStart(parentContext, request)) {
    return;
  }

  context = instrumenter().start(parentContext, request);
  scope = context.makeCurrent();
}

@Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)
public static void onExit(@Advice.Argument(1) Object request,
                          @Advice.Return Object response,
                          @Advice.Thrown Throwable exception,
                          @Advice.Local("otelContext") Context context,
                          @Advice.Local("otelScope") Scope scope) {
  if (scope == null) {
    return;
  }
  scope.close();
  instrumenter().end(context, request, response, exception);
}

Notice that the example above doesn't use Context.current(), but a Java8BytecodeBridge method instead. This is intentional: if you are instrumenting a pre-Java 8 library, inlining Java 8 default method calls (or static methods in an interface) into that library results in a java.lang.VerifyError at runtime, since Java 8 default method invocations aren't legal in Java 7 (and prior) bytecode.

Since the OpenTelemetry API has many common default/static interface methods, like Span.current(), the javaagent-extension-api artifact has a class Java8BytecodeBridge which provides static methods for accessing these default methods from advice. We suggest avoiding Java 8 language features in advice classes at all - sometimes you don't know what bytecode version is used by the instrumented class.

Associate instrumentation classes with instrumented library classes

Sometimes there is a need to associate some instrumentation class with an instrumented library class, and the library does not offer a way to do this. The OpenTelemetry javaagent provides VirtualField for that purpose. Consider the following example:

VirtualField<Runnable, Context> virtualField =
    VirtualField.get(Runnable.class, Context.class);

A VirtualField has a very similar interface to a map. It is not a simple map though: the javaagent uses many bytecode tweaks to optimize it. Because of this, retrieving a VirtualField instance is rather limited: the VirtualField#get() method must receive class references as its parameters; it won't work with variables, method params, etc. Both the owner class and the field class must be known at compile time for it to work.

Use of VirtualField requires the muzzle-generation gradle plugin. Failing to use the plugin will result in ClassNotFoundException when trying to access the field.

Avoid using @Advice.Origin Method

You shouldn't use ByteBuddy's @Advice.Origin Method method, as it inserts a call to Class.getMethod(...) in a transformed method.

Instead, get the declaring class and method name, as loading constants from a constant pool is a much simpler operation.

For example:

@Advice.Origin("#t") Class<?> declaringClass,
@Advice.Origin("#m") String methodName

Use non-inlined advice code with invokedynamic

Using non-inlined advice code is possible thanks to the invokedynamic instruction, this strategy is referred as "indy" in reference to this, by extension "indy modules" are the instrumentation modules using this instrumentation strategy.

The most common way to instrument code with bytebuddy relies on inlining, this strategy will be referred as "inlined" strategy in opposition to "indy".

Indy modules and transition

Having all instrumentation rely on native "indy" instrumentation is a tedious task and can't be achieved in a "big bang" step, we thus have to use intermediate steps.

Instrumentation modules that are "indy native" must have their InstrumentationModule#isIndyModule implementation return true. Also, all the instrumentation advice methods annotated with @Advice.OnMethodEnter or @Advice.OnMethodExit must have the inlined = false explicitly set.

The otel.javaagent.experimental.indy (default false) configuration option allows to opt-in for using "indy". When set to true, the io.opentelemetry.javaagent.tooling.instrumentation.indy.AdviceTransformer will transform advices automatically to make them "indy native".

This configuration is automatically enabled in CI when test indy label is added to a pull-request or when the -PtestIndy=true parameter is added to gradle.

In order to preserve compatibility with both instrumentation strategies, we have to omit the inlined = false from the advice method annotations.

We have two sets of instrumentation modules:

• "indy compatible": compatible with both "indy" and "inlined", do not override isIndyModule
• "inlined only": only compatible with "inlined", isIndyModule returns false.

The first step of the migration is to move all the "inlined only" to the "indy compatible" category by refactoring them with the limitations described below.

Once everything is "indy compatible", we can evaluate changing the default value of otel.javaagent.experimental.indy to true and make it non-experimental.

shared classes and common classloader

By default, all the advices of an instrumentation module will be loaded into isolated classloaders, one per instrumentation module. Some instrumentations require to use a common classloader in order to preserve the semantics of static fields and to share classes.

In order to load multiple InstrumentationModule implementations in the same classloader, you need to override the ExperimentalInstrumentationModule#getModuleGroup to return an identical value.

advice local variables

With inlined advices, declaring an advice method argument with @Advice.Local allows to define a variable that is local to the advice execution for communication between the enter and exit advices.

When advices are not inlined, usage of @Advice.Local is not possible. It is however possible to return a value from the enter advice and get the value in the exit advice with a parameter annotated with @Advice.Enter, for example:

@Advice.OnMethodEnter(suppress = Throwable.class)
public static Object onEnter(@Advice.Argument(1) Object request) {
  return "enterValue";
}

@Advice.OnMethodExit(suppress = Throwable.class, onThrowable = Throwable.class)
public static void onExit(@Advice.Argument(1) Object request,
                          @Advice.Enter Object enterValue) {
  // do something with enterValue
}

modifying method arguments

With inlined advices, using the @Advice.Argument annotation on method parameter with readOnly = false allows to modify instrumented method arguments.

When using non-inlined advices, reading the argument values is still done with @Advice.Argument annotated parameters, however modifying the values is done through the advice method return value and @Advice.AssignReturned.ToArguments annotation:

@Advice.OnMethodEnter(suppress = Throwable.class)
@Advice.AssignReturned.ToArguments(@ToArgument(1))
public static Object onEnter(@Advice.Argument(1) Object request) {
  return "hello";
}

It is possible to modify multiple arguments at once by using an array, see usages of @Advice.AssignReturned.ToArguments for detailed examples.

modifying method return value

With inlined advices, using the @Advice.Return annotation on method parameter with readOnly = false allows to modify instrumented method return value on exit advice.

When using non-inlined advices, reading the original return value is still done with @Advice.Return annotated parameter, however modifying the value is done through the advice method return value and @Advice.AssignReturned.ToReturned

@Advice.OnMethodExit(suppress = Throwable.class)
@Advice.AssignReturned.ToReturned
public static Object onEnter(@Advice.Return Object returnValue) {
  return "hello";
}

writing to internal class fields

With inlined advices, using the @Advice.FieldValue(value = "fieldName", readOnly = false) annotation on advice method parameter allows to modify the fieldName field of the instrumented class.

When using non-inlined advices, reading the original field value is still done with @Advice.FieldValue annotated parameter, however modifying the value is done through the advice method return value and @Advice.AssignReturned.ToFields annotation:

@Advice.OnMethodExit(suppress = Throwable.class)
@Advice.AssignReturned.ToFields(@ToField(value = "fieldName"))
public static Object onEnter(@Advice.FieldValue("fieldName") Object originalFieldValue) {
  return "newFieldValue";
}

It is possible to modify multiple fields at once by using an array, see usages of @Advice.AssignReturned.ToFields for detailed examples.