Java IO Basics: How to Download, Install, and Use Jar Files
How to Create and Run a Jar File in Java
A jar file, or Java ARchive, is a file format that allows you to bundle multiple Java class files and other resources into a single file. Jar files are useful for packaging and distributing Java applications, libraries, and applets. They also enable features such as compression, encryption, digital signing, and manifest specification.
In this article, we will learn how to create and run a jar file in Java using the jar and java commands. We will also see how to use external jar files as dependencies for our Java programs.
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Creating a Jar File
To create a jar file, we need to use the jar command, which is part of the Java Development Kit (JDK). The jar command has various options that allow us to customize the creation of the jar file. Here are some common options:
c: create a new jar file
f: specify the name of the jar file
e: set the entry point class for an executable jar file
m: include a manifest file with custom information
v: generate verbose output
u: update an existing jar file
Let's see some examples of using the jar command to create a jar file.
Using the Defaults
If we want to create a simple jar file with the default settings, we can use the c and f options. For example, if we have a directory called com/baeldung/jar that contains some Java class files, we can create a jar file called JarExample.jar with this command:
jar cf JarExample.jar com/baeldung/jar/*.class
This will create a jar file that contains all the class files in the directory. The jar file will also have a default manifest file that contains some basic information about the jar file.
Setting the Main Class
If we want to create an executable jar file, we need to specify the main class that will be invoked when we run the jar file. The main class is the one that has a public static void main(String args) method. We can use the e option to set the main class for our jar file. For example, if we have a class called JarExample that has a main method, we can create an executable jar file with this command:
jar cfe JarExample.jar com.baeldung.jar.JarExample com/baeldung/jar/*.class
This will create a jar file that contains all the class files in the directory and also sets the main class as com.baeldung.jar.JarExample. The jar command will add this information to the manifest file of the jar file.
Updating the Contents
If we want to update an existing jar file with new or modified files, we can use the u option. For example, if we have made some changes to our JarExample.class file and recompiled it, we can update our jar file with this command:
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jar uf JarExample.jar com/baeldung/jar/JarExample.class
This will replace the old JarExample.class file in the jar file with the new one.
Setting a Manifest File
If we want to have more control over what goes in our manifest file, we can provide our own manifest information in a text file. The manifest is a special file in a jar located in the META-INF directory. The manifest file has a name of MANIFEST.MF and contains information about the jar file and its contents. We can use the m option to specify our own manifest file. For example, if we have a file called mymanifest.txt that contains some custom information, we can create a jar file with this command:
jar cfm JarExample.jar mymanifest.txt com/baeldung/jar/*.class
This will create a jar file that contains all the class files in the directory and also uses our manifest file as the manifest for the jar file.
Running a Jar File
To run a jar file, we need to use the java command, which is part of the Java Runtime Environment (JRE). The java command has various options that allow us to customize the execution of the jar file. Here are some common options:
-jar: execute a jar file
-cp or -classpath: specify the classpath for finding classes and resources
-D: set a system property
-X: set non-standard options for the JVM
-verbose: enable verbose output
Let's see some examples of using the java command to run a jar file.
Running an Executable Jar File
If we have an executable jar file, we can run it with the -jar option. For example, if we have a jar file called JarExample.jar that has a main class specified in its manifest, we can run it with this command:
java -jar JarExample.jar
This will invoke the main method of the main class and execute the jar file.
Passing Arguments to the Main Method
If we want to pass some arguments to the main method of our jar file, we can do so after the name of the jar file. For example, if we want to pass two arguments "Hello" and "World" to our jar file, we can run it with this command:
java -jar JarExample.jar Hello World
This will pass the arguments as an array of strings to the main method of our jar file.
Setting System Properties
If we want to set some system properties for our jar file, we can use the -D option. For example, if we want to set a system property called myproperty with a value of myvalue, we can run our jar file with this command:
java -Dmyproperty=myvalue -jar JarExample.jar
This will set the system property before executing the jar file. We can access the system property in our Java code using the System.getProperty() method.
Tuning JVM Options
If we want to tune some JVM options for our jar file, we can use the -X option. For example, if we want to set the maximum heap size for our jar file to 256 MB, we can run it with this command:
java -Xmx256m -jar JarExample.jar
This will set the maximum heap size before executing the jar file. We can use other -X options to adjust other JVM parameters such as garbage collection, debugging, and performance.
Using External Jar Files
Sometimes, we may need to use external jar files as dependencies for our Java programs. For example, if we want to use a third-party library or framework that is packaged as a jar file, we need to add it to our classpath. The classpath is a list of locations where Java looks for classes and resources.
We can add external jar files to our classpath using the -cp or -classpath option of the java command. We can specify multiple jar files separated by a colon (:) on Linux and Mac OS or a semicolon (;) on Windows. For example, if we have two external jar files called JarLib1.jar and JarLib2.jar, we can add them to our class
This will display the names of all the files in the jar file.
How do I create a jar file from an Eclipse project?
You can use the Export wizard in Eclipse to create a jar file from an Eclipse project. To do this, follow these steps:
Right-click on your project in the Package Explorer and select Export.
Select Java > JAR file and click Next.
Choose the resources to export and the destination for the jar file.
Optionally, configure the options for the jar file, such as manifest, compression, and packaging.
Click Finish to create the jar file.
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