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|Developer(s)||Apache Software Foundation</td></tr>|
|Stable release||5.0 M12 / December 5, 2009</td></tr>|
|Written in||C++ and Java</td></tr>|
|Operating system||Windows and Linux</td></tr>|
|Type||Java Virtual Machine and Java Library</td></tr>|
|License||Apache License 2.0</td></tr>|
</table> Apache Harmony is an open source / free Java implementation, starting with Java SE 5.0. It will be licensed under the Apache License, Version 2. It was announced in early May 2005 and on October 25, 2006, the Board of Directors voted to make Apache Harmony a top-level project.
The Harmony project was initially conceived as an effort to unite all developers of the Free Java implementations. Many developers expected that it would be the project above the GNU, Apache and other communities. GNU developers were invited into and participated during the initial, preparatory planning. Later it was decided not to use the code from GNU Classpath, and that Harmony would use an incompatible license; therefore blocking the sharing of code between Harmony and existing free Java projects. Apache developers would then write the needed classes from scratch and expect necessary large code donations from software companies.
Reasons for rewriting from scratch
The main reason for disagreements between the GNU Classpath and Apache projects is due to differences between the GNU General Public License (GNU Classpath's license also contains the linking exception) and Apache License. These disagreements are inspired by various organizations and individuals that prefer the Apache License, which allows for closed source derived works. GNU Classpath can also be linked with proprietary code, but it is legally difficult to make a closed source derivative of GNU Classpath itself. However, many free software developers think that the licenses and philosophy of these communities are not different enough to split the efforts, and the inability to find compromises is frequently estimated negatively. The occasionally appearing suggestion to view these projects as opposing forces does not find wide support. Enthusiasts of the free software tend to break the discussion with the simple words that "more free software is not a problem".
Difficulties to obtain a TCK license from Sun
The Apache Software Foundation sent a letter to Sun Microsystems CEO, Jonathan Schwartz, on April 10, 2007, regarding their inability to acquire an acceptable license for the Java SE 5 Technology Compatibility Kit (TCK), a test kit needed by the project to demonstrate compatibility with the Java SE 5 specification, as required by the Sun specification license for Java SE 5. What makes the license unacceptable for ASF is the fact that it imposes rights restrictions through limits on the "field of use" available to users of Harmony, not compliant with the Java Community Process rules.
Sun answered on a company blog  that it intended to create an open source implementation of the Java platform under GPL, including the TCK, but that their current priority was to make the Java Platform accessible to the GNU/Linux community under GPL as quickly as possible.
This answer triggered some reactions, either criticizing Sun for not responding "in a sufficiently open manner" to an open letter , or rather Apache Software Foundation; some think that ASF acted unwisely to aggressively demand something they could have obtained with more diplomacy from Sun, especially considering the timescale of the opening class library  .
Use in Android SDK
Dalvik, the Java Virtual Machine used in Google's Android platform, use a subset of Harmony for the core of its Class Library. However, Dalvik does not align to Java SE nor Java ME Class Library profiles (for example J2ME classes, AWT or Swing are not supported). Instead it uses its own library, built on top of a subset of Harmony.
Apache Harmony started from being mostly developed by several companies, receiving large code contributions at once. However, the general discussions on the mailing lists were always opened for everyone, and later the Apache Software foundation mentors put a lot of effort to bring the development process more in line with the Apache way, and it seems that their efforts were highly successful. In November 1, 2006, the current team of committers consisted of 16 developers, 12 of them from IBM and Intel.
Recent development status
Apache Harmony was accepted among the official Apache projects on 29 October 2006.
The Dynamic Runtime Layer virtual machine consists of the following components:
Support Platform and Operating System
The project provide a portable implementation that ease development on many platforms and operating systems. The main focus is on Windows and Linux operating systems on x86 and x86-64 architectures.
Class Library Coverage
The part of the implemented classes is still smaller than in GNU Classpath (97.7% in the trunk versus almost 100% as of July 2007[update]), despite some non-trivial applications were shown being running in 2006 JavaOne international conference.
The progress of the Apache Harmony project can be tracked against J2SE 1.4 and Java SE 5.0. Also, there is a branch for Harmony v6.0 in development for Java SE 6.0.
Apache Harmony developers integrate several existing, field-tested open-source projects to meet their goal (not reinventing the wheel). Many of these projects are mature and well known and other part of the library need be writing from scratch.
This is a list of existing open source components that are used in the Apache Harmony project; some of them were in use before the project started.
Harmony is currently less documented than the alternative free Java implementations. For instance, in GNU Classpath every method of the central CORBA class (ORB) has the explaining comment both in the standard abstract API class  and implementation . In the Yoko project, used by Harmony , the majority of methods both in the standard declaration  and implementing class  were not documented (at the end of October, 2006). Also, GNU Classpath supported both older and current CORBA features (same as Sun's implementation). Harmony, differently, left the central method of the older standard (ORB.connect(Object)) fully unimplemented.
A complete implementation of the Java platform also requires a compiler that translates Java source code into bytecodes, a program that manages JAR files, a debugger, and an applet viewer and web browser plugin, to name a few. Harmony currently has the compiler, appletviewer, jarsigner, javah, javap, keytool, policytool, and unpack200 .
Virtual machine support
Harmony currently has seven virtual machine implementations that run Harmony Class Library, all of which were donations by external groups:
In the end of November, 2006, the language support provided by these virtual machine was still incomplete, and the build instructions recommended to use IBM's proprietary J9 instead to run the class library test suite. However, this is not necessary anymore (as of July 2007). The DRLVM virtual machine is currently (as of July 2006[update]) under heavy development, so a fast improvement of its features can be expected.
However, Harmony's incomplete library prevents it from launching some other applications: