You can use the loader.path parameter to define a location for an external lib folder. All jars under this folder will be added to the classpath. For example, if you'd like to define C:\extLib as your external lib folder, you can do the following:

java -Dloader.path=/C:/extLib/ -jar aapName.jar

For this to work, you need to use the PropertiesLauncher. There are two ways to do that:

Option 1

Update the project pom.xml and add the following tag:

<configuration>  <!-- added -->
  <layout>ZIP</layout> <!-- to use PropertiesLauncher -->
</configuration>

Effective build tag, the post-update looks like below:

<build> 
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>  <!-- added -->
                <layout>ZIP</layout> <!-- to use PropertiesLauncher -->
            </configuration>
        </plugin>
    </plugins>
</build>

Option 2

Use the PropertiesLauncher when launching the application from the commandline:

java -cp aapName.jar -Dloader.path=/C:/extLib/ org.springframework.boot.loader.PropertiesLauncher

References:
How to add jars to SpringBoot classpath with jarlauncher

On Linux:

Copyjava -cp MyApp.jar:/home/sleeper/thirdparty/lib -Dloader.main=myMainApplicationClass org.springframework.boot.loader.PropertiesLauncher

On Windows:

Copyjava -cp MyApp.jar;/home/sleeper/thirdparty/lib -Dloader.main=myMainApplicationClass org.springframework.boot.loader.PropertiesLauncher

This will avoid messing with the manifest or the Spring Boot Maven plugin configuration as in the other answers. It will launch your app with the PropertiesLauncher, which allows you to specify the main class in loader.main. As mentioned earlier, for some reason if you use PropertiesLauncher with loader.path, it will not add resource files to the classpath. This works around the issue by using -cp instead of -jar.

EDIT As mentioned by Pianosaurus in the comment, use ":" instead of ";" as separator in the classpath on Linux

Given that you are dealing with a read-only filesystem, using Spring Boot’s PropertiesLauncher may be your best option.

With the properties launcher you can use the loader.path system property to configure the classpath. BOOT-INF/classes and jars in BOOT-INF/lib are always included so loader.path should just point to the additional volume-mounted jar file.

You can configure Spring Boot to build a jar that uses properties launcher with Gradle and Maven.

You need to use PropertiesLauncher instead of JarLauncher:

java -cp "yourBoot.jar" -Dloader.path=plugins/ -Dloader.main=your.mainClass org.springframework.boot.loader.PropertiesLauncher

This will make java load the jars in the plugins directory and spring load all the classes in those jars marked as @Configuration into the context.

If you're using maven to build your application, it runs in the netbeans IDE, but you'll need to define a plugin in your maven pom.xml in order to include all of the required .jars in your final .jar to make it executable.

    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <executions>
            <execution>
                <phase>package</phase>
                <goals>
                    <goal>shade</goal>
                </goals>
                <configuration>
                    <transformers>
                        <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
                            <mainClass>yourmainpackagename.YourMainClass</mainClass>
                        </transformer>
                        <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                            <resource>META-INF/spring.handlers</resource>
                        </transformer>
                        <transformer implementation="org.apache.maven.plugins.shade.resource.AppendingTransformer">
                            <resource>META-INF/spring.schemas</resource>
                        </transformer>
                    </transformers>
                </configuration>
            </execution>
        </executions>
    </plugin>

This approach uses the maven shade plugin. You could also be using the maven assembly plugin, be prepared to face nasty bugs though :p

cheers.