Maven spring-boot：针对已编译的jar运行

Question

Is it possible to run maven, and specifically the spring-boot:run goal, directly against the jar file created as a result of the package goal as opposed to the source?

The reason I want to do this is because I want to run a spring boot app in a Docker container with hot swappable classes (stored on the host and mapped into the container using a shared volume) using spring boot Devtools (which sadly doesn't work if you run the application using Java -jar) and I don't want to put the source code for the app inside the container as well.

Answer 1

The classes parameter is not optional (it is typically inferred using the standard Maven project structure) but you could provide it with an empty directory and then include the previously packaged JAR using the folders parameter. Here's an example

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <mainClass>com.this.that.YourApplication</mainClass>
        <!-- any directory will do as long as (a) it exists and (b) it does not contain classes -->
        <classesDirectory>/tmp/</classesDirectory>
        <folders>
            <folder>
                <!-- the address, relative to the plugin's workingDirectory, of the 'original' application JAR -->
                tmp/lib/your-application.jar.original
            </folder>
        </folders>
    </configuration>
</plugin>

With this configuration, if you run with -X you'll see that the first two entries in the classpath for the JVM spawned by the Spring Boot plugin are (1) your application JAR and (2) the empty classes directory. For example:

[DEBUG] Classpath for forked process: <full_path_removed>/tmp/lib/your-application.jar.original:/tmp:

Notes:

• You'll need to supply the mainClass because you are using the original JAR which does not include the Main-Class instruction in META-INF/MANIFEST.MF
• You'll need to reference the "original" JAR rather than the Spring Boot packaged JAR because the original JAR is the one which includes your application's main class in its original location (the Spring Boot packaged JAR relocates it).