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jpf-nhandler is an extension of Java PathFinder (JPF). It automatically delegates the execution of SUT methods from JPF to the host JVM. Execution of a call o.m(a) delegated by jpf-nhandler follows three main steps:

  1. It transforms the JPF representation of o and a to the host JVM level.

  2. It delegates the execution to the original (non-native or native) method m by invoking it on the host JVM.

  3. Finally, it transforms the result of the method call back to its JPF representation.

The implementation of jpf-nhandler mostly relies on MJI. jpf-nhandler creates bytecode for native peers on-the-fly (they are called OTF peers from now on) using the BCEL library. To delegate the execution of a method to the host JVM, jpf-nhandler adds a method in the corresponding OTF native peer which implements the three steps described above.

The main applications of jpf-nhandler:

  1. The key application of jpf-nhandler is to automatically intercept and handle native calls within JPF. This extends the JPF functionality considerably, since it allows JPF to verify numerous SUTs on which JPF otherwise would crash.

  2. By using jpf-nhandler, rather than model checking a call, the call is executed outside of JPF, in its normal environment. Hence, this tool can be used to reduce the state space and improve the scalability of JPF.

  3. JPF creates execution traces as it runs the SUT. Long traces can cause JPF to run out of memory. In such cases, jpf-nhandler can be used to delegate methods with long traces, and execute them on the host JVM.

  4. Delegating a method may also speed up JPF.

jpf-nhandler can be configured in variety of ways. Here are some examples:

  • It can be used to skip calls instead of delegating them. In this case methods are executed as if they are empty and they just return some dummy value.

  • It also provides a way to specify which methods are delegated or skipped. To force JPF to delegate the constructor of the class a.b.C, use

    nhandler.spec.delegate = a.b.C.<init>

    To force JPF to delegate all method in the String class, use

    nhandler.spec.delegate = java.lang.String.*

    To force JPF to skip, use

    nhandler.spec.skip =
  • jpf-nhandler can also be configured to only delegate native calls which are not handled in JPF

  • jpf-nhandler can be also configured to generate source code for OTF peers on-the-fly, which allows the user to subsequently refine its implementation. Note that you can find bytecode and sources of OTF peers in the following directory.


    To generate sources, use

    nhandler.genSource = true

    If you refined and edited OTF sources and wish to compile them, run the following command from jpf-nhandler.

  • Since on-the-fly bytecode generation is expensive, one can also configure jpf-nhandler to retain and reuse OTF peers for future runs, i.e. their body may be extended as jpf-nhandler delegates more calls in the future.

    To reuse sources, use

    nhandler.clean = false

Limitations of jpf-nhandler

  1. The implementation of some classes is platform-specific, for instance java.lang.System. jpf-nhandler cannot be used for such classes due to inconsistencies between JPF and the host JVM.

  2. Since jpf-nhandler relies on transforming objects and classes between JPF and the host JVM, the state of a class or object should consist of the same fields and superclasses in both the host JVM and JPF. This limits the application of jpf-nhandler for types with JPF model classes that are inconsistent with the actual class in the Java library.

  3. The side effects of the delegated method should be only observable through the return value, the arguments of the method, and the object or class invoking the method, e.g. the lock() method in the class java.util.concurrent.locks.ReentrantLock cannot be handled.

  4. jpf-nhandler cannot handle certain objects of which part of their state is kept natively, e.g. java.awt.Window

Licensing of jpf-nhandler

This extension is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This extension is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You can find a copy of the GNU General Public License at

Installing of jpf-nhandler

To install jpf-nhandler, follow the steps below.

  1. Install JPF. See

  2. Install jpf-nhandler. The source for jpf-nhandler: See

  3. Build jpf-nhandler using ant. See

  4. Add jpf-nhandler to the file See

Running JPF with jpf-nhandler

To run JPF on the class Example the *.jpf file includes

@using = jpf-nhandler

target = Example

nhandler.delegateUnhandledNative = true

classpath = path-to-application-classes

native_classpath = path-to-application-classes

Note that to use jpf-nhandler, the classes used in the system under test should be specified both in classpath and native_classpath. Because The execution goes back and forth between JPF and the underlying host JVM, therefore both JPF and the host JVM should be able to access these classes.


Please email them to


to Peter Mehlitz for his help with the development of jpf-nhandler

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