In the field of software, JAR files (Java archive, English: Java? Archive) is a software package file format, which is usually used to aggregate a large number of Java class files, related metadata and resources (text, pictures, etc.). ) files into one file in order to develop Java platform applications or libraries.
JAR file is an archive file, which is in ZIP format. Jar as file extension. Users can create or extract JAR files by using jar commands included with JDK. You can also use other zip compression tools, but the order of entries in the zip file header is very important when compressing, because the manifest file usually needs to be put first. The file name in the JAR file is Unicode text.
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Java consists of four aspects: Java programming language, Java class file format, Java virtual machine and Java application program interface.
When editing and running a Java program, these four aspects should be involved simultaneously. Use word editing software (such as Notepad, WordPad, UltraEdit, etc.). ) or integrated development environment (Eclipse, MyEclipse, etc. Define different classes in Java source files.
Access the resource system by calling the methods in the class (these classes implement Java API), compile the source files into binary intermediate codes and store them in the class files, then run the class files by running the Java virtual machine corresponding to the operating system platform environment, execute the bytecode generated by compilation, and call the methods implemented in the class files to meet the Java API call of the program.
JAR file format provides many advantages and functions, many of which are not available in traditional compression formats such as ZIP or RAR. They include:
1, security. You can digitally sign the contents of the JAR file. In this way, the tool that can recognize the signature can selectively grant you software security rights, which other files can't do, and it can also detect whether the code has been tampered with.
2. Reduce the download time. If an applet is bundled into a JAR file, the browser can download the applet's class files and related resources in an HTTP transaction instead of opening a new connection for each file.
3. compression. JAR format allows files to be compressed to improve storage efficiency.
4. Transmission platform expansion. Java Extensions Framework provides a way to add functions to Java core platform, and these extensions are packaged with JAR files (Java 3D and JavaMail are examples of extensions developed by Sun).
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