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JAR (file format)
From Wikipedia, the free encyclopedia
| Java Archive | |
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| File extension: | .jar |
| MIME type: | application/java-archive |
| Uniform Type Identifier: | com.sun.java-archive |
| Developed by: | Sun Microsystems |
| Type of format: | file archive, data compression |
| Extended from: | ZIP |
In computing, a JAR file (or Java ARchive) is used for aggregating many files into one. It is generally used to distribute Java classes and associated metadata.
- WAR (file format) (Web Application aRchive) files are also Java archives which store XML files, java classes, Java Server Pages and other objects for Web Applications.
- RAR (Resource Adapter aRchive) files, not to be confused with the RAR file format, are also Java archives which store XML files, java classes and other objects for J2EE Connector Architecture (JCA) applications.
- EAR (file format) (Enterprise ARchive) files are composite Java archives which combine XML files, java classes and other objects including JAR, WAR and RAR Java archive files for Enterprise Applications.
Contents |
JAR files can be created and extracted using the "jar" command that comes with the JDK. It can be done using zip tools, but as WinZip has a habit of renaming all-uppercase directories and files in lower case, this can raise support calls with whoever created the JAR or the tool authors themselves. WinRAR, on the other hand, retains the original case of filenames.
A JAR file has an optional manifest file located in the path META-INF/MANIFEST.MF. The entries in the manifest file determine how the JAR file will be used. JAR files which are intended to be executed as standalone programs will have one of their classes specified as the "main" class. The manifest file would have an entry such as
Main-Class: myPrograms.MyClass
Such JAR files are typically started with a command similar to
java -jar foo.jar
These files can also include a Classpath entry, which identifies other JAR files to be loaded with the JAR. This entry consists of a list of absolute or relative paths to other JAR files. Although intended to simplify JAR use, in practice, it turns out to be notoriously brittle as it depends on all the relevant JARs being in the exact locations specified when the entry-point JAR was built. To change versions or locations of libraries, a new manifest is needed.
A JAR file can be digitally signed. If so, the signature information is added to the manifest file. The JAR itself is not signed, but instead every file inside the archive is listed along with its checksum; it is these checksums that are signed. Multiple entities may sign the JAR file, changing the JAR file itself with each signing -but the signed files themselves remain valid. When the Java runtime loads signed JAR files, it can validate the signatures and refuse to load classes that do not match the signature. It can also support 'sealed' packages, in which the classloader will only permit Java classes to be loaded into the same package if they are all signed by the same entities. This prevents malicious code from being inserted into an existing package, and so gain access to package-scoped classes and data.
JAR files can be obfuscated so that a user of the JAR file doesn't get much information regarding the code it contains, or to reduce its size, which is useful in mobile phone applications.
For those Microsoft Windows users who prefer having Windows EXE files, tools such as JSmooth or Launch4J can be used to wrap JAR files into executables. Eclipse uses a small EXE launcher (eclipse.exe) to display the splash screen on startup and launch the application from the main JAR (startup.jar).
The Apache Ant build tool has its own package to read and write the Zip and JAR archives, including support for the Unix filesystem extensions. The org.apache.tools.zip package is released under the Apache Software Foundation license and is designed to be usable outside Ant. This code is fast and widely used. It creates most JAR files that are not created with Sun's utility, so could be considered fairly mature.
The Ant team found that most of their support calls related to JAR file creation have two underlying causes.
- The first is manifest creation, specifically how long lines in the manifest are wrapped. This is a complex and somewhat ambiguous part of the specification. Ant wraps long lines at 68 characters and continues on the following line with a space at the front to indicate a continuation. This is viewed as erroneous by people that have not read the specification in detail and believe that the classpath should be split at a file boundary, instead of partly across a file name. Unfortunately, if that is done, the Java runtime does not detect a split line as the first line ends before the 68 character boundary.
- The second is WinZip converting upper-case files and directories to lower case. If a user views the contents of a JAR file using WinZip, a file such as MANIFEST/MANIFEST.MF is converted to manifest/manifest.mf. Some mobile phone Java runtimes appear to parse the manifest in ways that are incompatible with the specification, and require a strict ordering of entries in the manifest. They also do not implement the line wrapping algorithm correctly. This may imply a problem in the test suite for the J2ME mobile java runtime.
- JAR File Specification
- Using JAR files: The Basics (from Sun)
- Lesson: Packaging Programs in JAR Files (from Sun)
- Jar File Overview (from Sun)
- Jar Class Loader (dynamically loading classes directly from Jar files)
- fastjar - alternative .jar creation program written in C
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| Archiving only | ar · cpio · shar · tar · LBR |
| Compression only | bzip2 · gzip · LZMA · lzop · rzip · SQ · compress · LZW |
| Archiving and compression | 7z · ACE · ARC · ALZip · ARJ · Cabinet · cpt · DAR · dd · DGCA · .dmg · GCA · kgb · LHA · LZX · PAQ · RAR · qda · sit · SQX · zoo · ZIP |
| Software packaging and distribution | deb · pkg · RPM · MSI · JAR (WAR · RAR (Java) · EAR) |