What is byte-monkey

Byte-Monkey is a small Java library for testing failure scenarios in JVM applications - it works by instrumenting application code on the fly to deliberately introduce faults like exceptions and latency. Original blogpost here.

Original git repo
Forked one to do short-circuit testing

How to use

java -javaagent:byte-monkey.jar=mode:fault,rate:0.5,filter:uk/co/probablyfine/ -jar your-java-app.jar
  • mode: What mode to run in - currently supports fault, latency, nullify, scircuit, and analyzetc. Default is fault
  • rate: Value between 0 and 1 - how often to activate the fault. Default is 1, i.e. 100%
  • filter: Only instrument packages or methods matching the (java-style) regex. Default is .*, i.e. all methods

byte-monkey is configured with a comma-separated key-value pair string of the options as the agent argument.

The example above would run in fault mode, activating on 50% of eligible method calls, for anything in the package tree below uk.co.probablyfine

When you want to analyze the try-catch blocks info, you can also set byte-monkey into pom.xml, like this:

                <!-- Java 7 introduced stricter verification and changed the class format a bit—to contain a stack map used to verify that code is correct. -->
                <!-- Java version or bytecode instrumentation could both be to blame. -->
                <!-- So we just close verify to make our short-circuit testing work -->
                <argLine>-noverify -javaagent:byte-monkey.jar=mode:analyzetc,filter:your/root/package/name</argLine>

With this configuration, when you run mvn test, byte-monkey will detect all the try-catch blocks covered by test cases and inject a logging method into try block. So that you can get some logs to help your analysis. Logs are something like this:

INFO ByteMonkey try catch index 0 @ L1289479439, copyPropertiesWithExclude @ com.github.sworm.spojo.utils.SpojoUtils
INFO ByteMonkey testCase: copyInclude2ndLevelComplexProperty @ com.github.sworm.spojo.utils.SpojoUtilsMoreTest

L1289479439 is a relative position of the try-catch block, because we analyze this from java byte-code, index 0 indicates the number of this try block in the method. As for 1 try with n catch blocks, the index number will increase but the position stays the same. You can use Spoon together to do more analysis, like the percentage of try-catch covered by test cases. With this data, we might detect more bugs from those try-catch blocks which are not covered by test cases. We can also do the specific exception injection because we have obtained all the try-catch positions and exception types.

As for specific fault injection, you can use the following command:

java -javaagent:byte-monkey.jar=mode:scircuit,filter:package/path/ClassName/MethodName,tcindex=0 -jar your-java-app.jar

You can declare the exception type using tcindex, for example, the try block is corresponding to 2 catch blocks, then tcindex 0 indicates the first type of exception in the catch block.



Running byte-monkey in fault mode will cause the first declared exception in a method signature to be thrown.

CAVEAT: Byte-Monkey can only create Exceptions that expose a public default constructor as a result of how it instantiates them. If such a constructor doesn't exist, it falls back to a ByteMonkeyException instead.


Running byte-monkey in latency mode will cause the method to sleep before executing further instructions.

There is a configuration option available only during this mode:

  • latency: Duration (in millis) to wait on method calls, only valid when running in Latency mode. Default is 100ms

Example: java -javaagent:byte-monkey.jar=mode:latency,rate:0.5,latency:150 -jar your-java-app.jar


Running byte-monkey in nullify mode will replace the first non-primitive argument to the method call with a null value.

Methods with only primitive arguments or no arguments at all will not be affected by the agent in this mode.


Throw corresponding exceptions in the very beginning of try blocks.
What is short-circuit testing? What is this used for? You can read this paper or my article first.

Try-catch Analysis

Before we do short-circuit testing, we should analyze the application first, basically about how many try-catch blocks are covered by test cases. Then we can choose a specific one to inject exception and observe the results of all test cases.

Implementation Details

Byte-Monkey uses the JVM Instrumentation API. Implementing the API enables you to register transformers that can iterate over (and transform) class files as they are loaded by the JVM. Byte-Monkey uses Objectweb ASM which comes packaged with the JDK to chance the underlying bytecode of loaded classes



Different operation mode overrides the generateByteCode() method, in order to inject relevant codes, key codes are as below:

Mode: fault

final List<String> exceptionsThrown = method.exceptions;

InsnList list = new InsnList();

if (exceptionsThrown.size() == 0) return list;

list.add(new LdcInsnNode(exceptionsThrown.get(0)));
list.add(new MethodInsnNode(
    false // this is not a method on an interface

list.add(new InsnNode(Opcodes.ATHROW));

Mode: latency

list.add(new LdcInsnNode(arguments.latency()));
list.add(new MethodInsnNode(Opcodes.INVOKESTATIC, "java/lang/Thread", "sleep", "(J)V", false));

Mode: nullify

final Type[] argumentTypes = Type.getArgumentTypes(method.desc);

final OptionalInt firstNonPrimitiveArgument = IntStream
    .range(0, argumentTypes.length)
    .filter(i -> argumentTypes[i].getSort() == Type.OBJECT)

if (!firstNonPrimitiveArgument.isPresent()) return list;

list.add(new InsnNode(Opcodes.ACONST_NULL));
list.add(new VarInsnNode(Opcodes.ASTORE, firstNonPrimitiveArgument.getAsInt() + 1));

Design a new mode to do short-circuit testing

Based on Instrumentation API and byte-monkey's codes, mainly update the ByteMonkeyClassTransformer to support new mode "scircuit", and add new code snippet into enum OperationMode.

  • In ByteMonkeyClassTransformer: find the right position (beginning of try blocks) to inject codes
switch (failureMode) {
    case SCIRCUIT:
        int tcIndex = arguments.tcIndex();
        if (tcIndex < 0) {
                    .filter(method -> !method.name.startsWith("<"))
                    .filter(method -> filter.matches(cn.name, method.name))
                    .filter(method -> method.tryCatchBlocks.size() > 0)
                    .forEach(method -> {
                        // inject an exception in each try-catch block
                        // take the first exception type in catch block
                        // for 1 try -> n catch, we should do different injections through params
                        LabelNode ln = method.tryCatchBlocks.get(0).start;
                        int i = 0;
                        for (TryCatchBlockNode tc : method.tryCatchBlocks) {
                            if (ln == tc.start && i > 0) {
                                // if two try-catch-block-nodes have the same "start", it indicates that it's one try block with multiple catch
                                // so we should only inject one exception each time
                            InsnList newInstructions = failureMode.generateByteCode(tc, tcIndex, arguments);
                            method.maxStack += newInstructions.size();
                            method.instructions.insert(tc.start, newInstructions);
                            ln = tc.start;
        } else {
            // should work together with filter, inject an exception into specific position
  • In OperationMode: generate exception throwing method. DirectlyThrowException is a method who throws an exception, the type of the exception is defined in its parameter.
public InsnList generateByteCode(TryCatchBlockNode tryCatchBlock, int tcIndex, AgentArguments arguments) {
    InsnList list = new InsnList();

    list.add(new LdcInsnNode(tryCatchBlock.type));
    list.add(new MethodInsnNode(
        false // this is not a method on an interface
    return list;


  • How to handle try-catch nesting? Do we need to define a testing level?


  • Handle one try with multiple catch blocks

In this case, the test case should be excuted for more times, every time different exception is injected into the try block.
Byte-monkey can support more parameters in short-circuit mode now, besides filter, we can use tcindex to declare the specific type of exception you want to inject.

  • Integrate with Maven/JUnit/TestNG... to do automatic testing

This is mainly used for try-catch analysis, but the most important goal for us is chaos engineering, i.e. injecting kinds of exceptions into production environment and learning from the system's reaction, to help build the confidence of system behavior.

  • Define a injection rate in short-circuit mode

标签: Fault Injection, Experiment, Byte-monkey