How to run jar package in java?

How to run jar package in java: (Recommended: java video tutorial)

Everyone knows that a java application project can be packaged into a jar, of course you must specify a main class with a main function as the program entry point of your jar package.

The specific method is to modify the MANIFEST.MF file under the META-INF directory in the jar package.

For example, there is a jar package called test.jar, which contains a main class with a main function: test.someClassName

We only need to add the following sentence to MANIFEST.MF:

Main-Class: test.someClassName

Then we can enter java -jar test.jar in the console to run the jar.

Methods to run third-party jar packages:

Method 1. Use Bootstrap Classloader to load these classes

We can use the following parameters at runtime :

-Xbootclasspath: Completely replaces the system Java classpath. It is best not to use it.
-Xbootclasspath/a: Loaded after the system class is loaded. Generally use this.
-Xbootclasspath/p: Load before loading the system class, pay attention to its use, it will not be good if it conflicts with the system class.

win32 java -Xbootclasspath/a: some.jar;some2.jar; -jar test.jar
unix    java -Xbootclasspath/a: some.jar:some2.jar: -jar test.jar

Each jar is separated by a semicolon in the win32 system, and a colon is used in the unix system. Separate

Method 2. Use Extension Classloader to load

You can throw all the jars that need to be loaded into %JRE_HOME%/lib/ext, in this directory The jar package will be loaded by Extension Classloader after Bootstrap Classloader completes its work. Very convenient and very worry-free. :)

Method three, still use AppClassloader to load, but no classpath parameter is required

We add the following code in MANIFEST.MF:

Class-Path: lib/some.jar

lib is a subdirectory in the same directory as test.jar, and the some.jar package to be referenced by test.jar is in it.

If there are multiple jar packages that need to be referenced:

Class-Path: lib/some.jar lib/some2.jar

Use spaces for each individual jar Just separate. Be careful to use relative paths.

Method 4. Customize Classloader to load

This method is the ultimate solution. Basically, all well-known java applications do this, such as tomcat and jboss etc.

For more java knowledge, please pay attention to the java basic tutorial column.

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