How to correctly load external JAR packages when Spring Boot project starts?

How to load external jar packages when spring boot project starts?

When building a complex spring boot application, in order to meet the needs of different customers and improve the maintainability of the code, it is often necessary to split the project into multiple modules, each module being packaged into an independent jar package. This article will discuss how to dynamically load these external jar packages when a main spring boot project is started.

The article mentions a scenario: a spring boot project is used to scan QR codes to query information, and supports various types such as trees, roads, items, etc. In order to decouple, the core functional modules are packaged into a jar package to provide basic services, while different types of information processing are packaged into independent jar packages respectively. When deploying, you only need to load the corresponding type of jar package.

I tried several ways to load external jar packages, such as using java -xbootclasspath/a, java -djava.ext.dirs, and java -dloader.path, but none of them succeeded. The java -xbootclasspath/a:file:./libs -jar main.jar command does not load the jar package in the libs directory; using java -djava.ext.dirs and java -dloader.path results in a startup exception, prompting that the main class cannot be found.

The key to the problem lies in the use of the -jar parameter. The -jar parameter directly specifies the running jar package and ignores the -classpath or -cp parameters. Therefore, it is necessary to bypass the -jar parameter and directly specify the main class and classpath.

The correct startup command is as follows:

 java -classpath './libs*:main.jar' org.springframework.boot.loader.launch.JarLauncher
# or
java -cp './libs*:main.jar' org.springframework.boot.loader.launch.JarLauncher

Here, ./libs* represents all jar packages in the libs directory, and main.jar is the jar package of the main spring boot application. org.springframework.boot.loader.launch.jarlauncher is the actual main class of the jar package after spring boot. In this way, all jar packages in the libs directory can be successfully loaded and the spring boot application can be run. It should be noted that this method requires explicitly specifying the startup class of the spring boot application.

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