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, java.io, 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.
The role of rt.jar in the Java runtime environment
The Java runtime environment consists of two main components: the JVM and the JRE. The JVM is the engine that executes the Java bytecode, which is the intermediate representation of the Java source code. The JRE is the set of libraries and tools that support the execution of Java applications.
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rt.jar is part of the JRE and provides the core classes that are loaded by the JVM at runtime. These classes implement the basic functionality of the Java platform, such as input/output, networking, security, reflection, concurrency, and so on. They also define the interfaces and abstract classes that are extended and implemented by other libraries and frameworks.
The dependency of Java applications on rt.jar
Any Java application that uses the standard Java API depends on rt.jar. This means that if you want to run a Java application on a different machine or platform, you need to have rt.jar available on that machine or platform. Otherwise, you will get a NoClassDefFoundError or a ClassNotFoundException, which indicate that the JVM could not find or load a class that was referenced by your application.
To avoid this problem, you can either bundle rt.jar with your application or ensure that the target machine or platform has a compatible JRE installed. However, bundling rt.jar can increase the size of your application and introduce compatibility issues if you use different versions of rt.jar. On the other hand, relying on the target machine or platform to have a compatible JRE can limit your portability and flexibility.
The benefits and drawbacks of using rt.jar
Using rt.jar has some advantages and disadvantages that you should be aware of. Here are some of them:
Benefits
Drawbacks
It provides a standard and consistent API for Java applications.
It simplifies the development and deployment of Java applications.
It ensures compatibility and interoperability among different Java platforms and versions.
It supports backward compatibility and security updates.
It increases the memory footprint and startup time of Java applications.
It contains some unused or deprecated classes that can cause performance or security issues.
It limits the customization and optimization of Java applications.
It creates dependency and compatibility issues among different Java platforms and versions.
How to download, update, and fix rt.jar?
If you need to download, update, or fix rt.jar, here are some steps that you can follow:
How to download rt.jar?
The easiest way to download rt.jar is to download and install a JDK or JRE that contains it. You can find various versions of JDK or JRE from Oracle's website or from other vendors such as OpenJDK, AdoptOpenJDK, Amazon Corretto, etc. You can also use tools such as SDKMAN or Jabba to manage multiple versions of JDK or JRE on your machine.
If you only need rt.jar, you can also download it separately from some online sources such as Maven Central or GitHub. However, be careful about the source and version of rt.jar, as they may not be official or compatible with your JDK or JRE.
How to update rt.jar?
The best way to update rt.jar is to update your JDK or JRE to a newer version that contains an updated version of rt.jar. This will ensure that you get the latest features, bug fixes, and security patches for your Java platform. You can check for updates from Oracle's website or from other vendors' websites. You can also use tools such as SDKMAN or Jabba to switch between different versions of JDK or JRE on your machine.
If you only need to update rt.jar, you can also download a newer version of rt.jar from some online sources such as Maven Central or GitHub. However, be careful about the source and version of rt.jar, as they may not be official or compatible with your JDK or JRE. You can also use tools such as JarCompare or JarDiff to compare and merge different versions of rt.jar.
How to fix rt.jar missing or corrupted error?
If you encounter an error that says rt.jar is missing or corrupted, it means that your JDK or JRE cannot find or load rt.jar. This can happen due to various reasons, such as incorrect installation, configuration, or update of your JDK or JRE, accidental deletion or modification of rt.jar, malware infection, disk corruption, etc.
To fix this error, you can try the following solutions:
Reinstall or repair your JDK or JRE.
Restore rt.jar from a backup or a copy.
Download a new version of rt.jar from a trusted source.
Scan and clean your system for malware and disk errors.
Check and fix your classpath and environment variables.
Conclusion
rt.jar is a vital component of the Java platform that contains all the core classes for the Java runtime environment. It provides the basic functionality and libraries that are required by the Java virtual machine and the Java compiler. It also ensures compatibility and interoperability among different Java platforms and versions.
rt.jar was present in all JDK releases until JDK 8, but it was removed in JDK 9 as part of the modularization of the Java platform. The classes that were previously in rt.jar are now stored in a more efficient format in the $JAVA_HOME/lib/modules file.
If you need to download, update, or fix rt.jar, you can follow the steps that we have outlined in this article. However, be careful about the source and version of rt.jar, as they may not be official or compatible with your JDK or JRE.
FAQs
What is the difference between rt.jar and tools.jar?
tools.jar is another JAR file that contains some tools and utilities for the Java development environment. It includes classes for the Java compiler, debugger, launcher, document generator, etc. Unlike rt.jar, which is part of the JRE, tools.jar is part of the JDK. Therefore, you need to have a JDK installed to use tools.jar.
How can I view the content of rt.jar?
You can view the content of rt.jar by using any ZIP tool that can open JAR files. You can also use the command-line tool jar, which is included in the JDK. For example, you can use the following command to list the content of rt.jar:
$ jar tf $JAVA_HOME/jre/lib/rt.jar META-INF/ META-INF/MANIFEST.MF java/ java/applet/ java/applet/Applet.class java/applet/AppletContext.class ...
How can I create my own rt.jar?
You can create your own rt.jar by using the command-line tool jar, which is included in the JDK. For example, you can use the following command to create a custom rt.jar that contains only the classes from the java.lang, java.util, and java.io
packages:
$ jar cf my_rt.jar -C $JAVA_HOME/jre/lib/rt.jar java/lang -C $JAVA_HOME/jre/lib/rt.jar java/util -C $JAVA_HOME/jre/lib/rt.jar java/io $ jar tf my_rt.jar META-INF/ META-INF/MANIFEST.MF java/ java/lang/ java/lang/AbstractMethodError.class java/lang/Appendable.class ... java/util/ java/util/AbstractCollection.class java/util/AbstractList.class ... java/io/ java/io/BufferedInputStream.class java/io/ BufferedOutputStream.class ...
However, be careful when creating your own rt.jar, as it may not be compatible with your JDK or JRE, or with other Java applications that depend on rt.jar.
How can I replace rt.jar with modules?
If you are using JDK 9 or later, you can replace rt.jar with modules by using the --module-path and --add-modules options of the Java launcher. For example, you can use the following command to run a Java application that requires only the java.base, java.logging, and java.xml modules:
$ java --module-path $JAVA_HOME/jmods --add-modules java.base,java.logging,java.xml -jar my_app.jar
This will load only the specified modules from the $JAVA_HOME/jmods directory, which contains the modular JAR files for the Java platform. You can also use the --list-modules option to list all the available modules in a given module path.
How can I check the version of rt.jar?
You can check the version of rt.jar by using the command-line tool jar, which is included in the JDK. For example, you can use the following command to extract and read the manifest file of rt.jar:
$ jar xf $JAVA_HOME/jre/lib/rt.jar META-INF/MANIFEST.MF $ cat META-INF/MANIFEST.MF Manifest-Version: 1.0 Created-By: 1.8.0_301 (Oracle Corporation) Specification-Title: Java Platform API Specification Specification-Version: 1.8 Specification-Vendor: Oracle Corporation Implementation-Title: Java Runtime Environment Implementation-Version: 1.8.0_301-b09 Implementation-Vendor: Oracle Corporation Implementation-Vendor-Id: com.oracle Extension-Name: java_runtime_environment Extension-Installation: jre/lib/rt.jar
The manifest file contains some information about the specification and implementation of rt.jar, such as the title, version, vendor, and extension name. You can use this information to identify the version of rt.jar. 44f88ac181