Why Is My Spring Boot App Slower with java -jar Than with mvn spring-boot:run?

If you’ve noticed that your Spring Boot application starts up much faster when you use mvn spring-boot:run (say, in 5 seconds) compared to java -jar myapp.jar (maybe 25 seconds), you’re not imagining it. This difference in startup time is common, and in this post, I’ll explain why it happens and how you can speed up your jar file execution to match Maven’s performance.

Why the Startup Time Differs

The way you launch your Spring Boot app affects how it starts, and here’s the breakdown:

• mvn spring-boot:run: This uses the Spring Boot Maven plugin. By default, it might run your app as a thread within Maven’s existing JVM (Java Virtual Machine) if configured with fork=false. Starting a thread is fast. However, if fork=true (the default), it launches a new JVM process—just like java -jar—but other factors can still make it quicker.
• java -jar myapp.jar: This always starts a fresh JVM process from scratch. Creating a new process involves more overhead, like memory allocation and loading the jar file, which slows things down.

In short: a thread (Maven, if not forking) starts faster than a full process (jar).

Key Reasons mvn spring-boot:run Is Faster

Here’s why Maven often outpaces the jar approach:

1. Thread vs. Process:

• With fork=false in Maven, your app runs inside Maven’s JVM as a thread, skipping the heavy lifting of launching a new process.
• java -jar always spawns a new process, which requires the operating system to allocate resources—a slower task.

1. Class Loading Efficiency:

• Maven might reuse pre-loaded classes or cached dependencies from its environment, cutting startup time.
• A jar file starts cold, loading all classes and dependencies anew, which takes longer—especially for big apps with many libraries.

1. JVM Warm-Up:

• If you’ve run mvn spring-boot:run before, the JVM might already be “warm,” with optimizations like JIT (Just-In-Time) compilation ready to go.
• Each java -jar run begins with a cold JVM, meaning no prior optimizations are in place.

How to Make java -jar as Fast as Maven

You can shrink the startup gap with these practical steps:

1. Use Layered Jars

Spring Boot 2.3+ introduced layered jars, which organize dependencies for faster loading. Build your jar with:

mvn spring-boot:repackage

Then run it:

java -jar myapp.jar

This speeds up class loading by separating your app’s code from its dependencies.

2. Match JVM Settings

Maven might use tuned JVM arguments (like heap size or compilation settings). Check your Maven setup and apply the same to your jar. For example:

java -Xmx512m -XX:+TieredCompilation -jar myapp.jar

To see Maven’s settings, look in your pom.xml under the Spring Boot plugin or check the console output when running mvn.

3. Streamline App Startup

Examine your app’s initialization. Are there slow tasks (like database connections) you can delay or optimize? For instance, use lazy loading for beans or tweak connection pooling settings.

4. Go Native (Optional)

For blazing-fast startup (think milliseconds), compile your app into a native image using GraalVM. This is more complex and suits production use, but it eliminates JVM startup overhead entirely.

Checking Maven’s Fork Setting

Maven’s speed advantage might hinge on its fork configuration. Look in your pom.xml:

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <fork>false</fork>  <!-- Faster: runs in Maven’s JVM -->
    </configuration>
</plugin>

If <fork>true</fork> (or it’s omitted, as true is the default), Maven starts a new JVM, so the speed difference might stem from class loading or disk performance instead.

Wrapping Up

The slower startup of java -jar myapp.jar compared to mvn spring-boot:run usually boils down to launching a new JVM process versus running in an existing one. By using layered jars, aligning JVM arguments, and optimizing your app’s startup logic, you can make your jar nearly as fast as Maven. For the ultimate boost, consider native images with GraalVM.

If you’re still stuck, let me know in the comments—I’d love to help troubleshoot!