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rt.jar: A Comprehensive Guide and Download Link

What is rt.jar and why do you need it?

If you are a Java developer or user, you might have encountered the term rt.jar at some point. But what is it exactly and why is it important for your Java applications? In this article, we will explain what rt.jar is, what it does, how it has changed over time, and how to download, update, and fix it.

What is rt.jar?

rt.jar stands for runtime JAR and contains all the compiled class files for the base Java Runtime Environment (JRE). It also includes the bootstrap classes, which are the core classes that comprise the Java platform API. In other words, rt.jar contains all the classes that are essential for running any Java application.

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The meaning of rt.jar

The name rt.jar is a bit cryptic, but it has a simple explanation. The rt part stands for runtime, which refers to the JRE that executes Java applications. The .jar part stands for Java ARchive, which is a file format that stores multiple files in a compressed form. A JAR file can also contain a manifest file, which can specify information such as the main class, the classpath, and the version of the JAR file.

The location and content of rt.jar

In Windows and Linux, rt.jar is usually located under $JAVA_HOME/jre/lib, where $JAVA_HOME refers to the JDK or JRE installation directory. You can open rt.jar with any ZIP tool and see its content. You will find that it contains all the packages from the Java API, such as java.lang, java.util,, and so on. It also contains some internal classes from the com, sun, and org packages, which are not meant for public use.

In MacOSX, prior to JDK 1.7, rt.jar was called classes.jar and was located under /System/Library/Frameworks//Classes. However, since JDK 1.7, Apple has stopped distributing Java and Oracle's JDK has adopted the same name and location as Windows and Linux.

The changes and removal of rt.jar in later JDK releases

rt.jar was present in all JDK releases until JDK 8. However, starting from JDK 9, rt.jar was removed as part of the modularization of the Java platform. Instead of storing all the classes in one large JAR file, they were organized into smaller modules that could be loaded on demand. This improved the performance, security, and maintainability of the Java platform.

The classes that were previously in rt.jar are now stored in a more efficient format in implementation-specific files in the $JAVA_HOME/lib/modules file. This file can be accessed by using the JRT file system, which is a virtual file system that allows you to browse and read the modules as if they were regular files.

Why do you need rt.jar?

rt.jar is essential for any Java application because it provides the basic functionality and libraries that are required by the Java Virtual Machine (JVM) and the Java compiler. Without rt.jar, you would not be able to use the basic features of the Java language, such as strings, arrays, exceptions, threads, collections, streams, and many more.

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