thingtono / faketime

Java agent to enable changing time in a Java project, mainly for testing.

Faketime

This native Java Agent allows you to change the time in a Java program, without affecting the system clock. It allows control over the System.currentTimeMillis() method, which is used by most other time-related functionality provided by the JVM.

How to use it

The easy way - For the Mac

• Download libfaketime.jnilib into some directory, say, /path/to/libfaketime.jnilib.

• Run your Java program (say, org.test.Main) with the agent-specific extra arguments (see issue #3), like this:

    java -agentpath:/path/to/libfaketime.jnilib \
      -XX:+UnlockDiagnosticVMOptions \
      -XX:DisableIntrinsic=_currentTimeMillis \
      -XX:CompileCommand=exclude,java/lang/System.currentTimeMillis \
      org.test.Main
  

• In your Java code, you can set the property faketime.offset.seconds to the number of seconds you want the time altered by. For example, to add a day, you can do something like this:

    System.setProperty("faketime.offset.seconds", "86400");
  

• That's it! Take a look at FakeTimeTest.java if you need to see some Java code which uses it.

The "hard" way - For the Mac

• Clone this repo

• Run this:

    gcc -shared -I $JAVA_HOME/include -Wall src/FakeTimeAgent.c -o libfaketime.jnilib
  

• This will create libfaketime.jnilib in the current directory. Run your Java program (say, org.test.Main) with the agent-specific extra arguments, like this:

    java -agentpath:/path/to/libfaketime.jnilib \
      -XX:+UnlockDiagnosticVMOptions \
      -XX:DisableIntrinsic=_currentTimeMillis \
      -XX:CompileCommand=exclude,java/lang/System.currentTimeMillis \
      org.test.Main
  

  or, if the current directory has libfaketime.jnilib, you can use:

    java -agentlib:faketime \
      -XX:+UnlockDiagnosticVMOptions \
      -XX:DisableIntrinsic=_currentTimeMillis \
      -XX:CompileCommand=exclude,java/lang/System.currentTimeMillis org.test.Main
  

• In your Java code, you can set the property faketime.offset.seconds to the number of seconds you want the time altered by. For example, to add a day, you can do something like this:

    System.setProperty("faketime.offset.seconds", "86400");
  

• That's it! Take a look at FakeTimeTest.java if you need to see some Java code which uses it.