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Java Bytecode Engineering Examples & Tutorials

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jon-bell/bytecode-examples

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Java Bytecode and JVMTI Examples

1. Method Coverage recording – efficiently records per-method coverage information (e.g. which methods in an application under test are executed by each test). Byte code is instrumented dynamically as its loaded into the JVM (using a java agent). There is a local cache within each class that records whether a method was hit during the current test case, and a global collection that stores them too. This local + global cache is much more performant than just keeping a global cache, because when each method is executed we can first check a local boolean field (which is easily optimized by the JIT compiler), and if it hasn’t been hit, THEN we store the fact that the method was executed in a global set (which is relatively much more expensive).
2. Static Instrumenter – applies byte code instrumentation statically, rather than at load time. This technique is needed if you want to instrument various core JRE classes that would be loaded already (and immutable) after your javaagent gets called.
3. Heap Tagging – uses JVMTI and byte code instrumentation to allow you to apply an arbitrary object “label” to every reference type (objects or arrays). Doing this for many instances of classes (objects) is trivial: we just add a field to each class to store the tag, and generate some code to set and fetch it for each class (every class is made to implement the interface Tagged). However, you can’t do this for all classes – the constant pool offsets for some fields of some classes (like Object, Long, Byte, etc.); plus you can’t do this for arrays (which aren’t instances of classes). For this, we use JVMTI’s getTag and setTag functions. Tagger provides an abstraction to get and set the label of an object. The JVMTI code implementation is mostly book-keeping that makes sure that we don’t leak memory from these object labels. The JVMTI code is largely inspired by another excellent example by Kelly O’Hair.
4. Heap Walking – uses JVMTI for a slightly contrived (but still somewhat interesting) example of heap walking and tagging. It crawls the heap (using FollowReferences), and for every object, builds a list of the static fields that can reach that object. After crawling, the library can return the list of static fields that point (perhaps indirectly) to the requested object. This example also shows off how to calculate the internal JVM field offsets for classes (which was a pain to write out my first time. ). Let me know (email, twitter, or comment on my blog post) if you have any questions or requests.