Fixing the ‘Sharing is only supported for boot loader classes’ Warning in Spring Boot Tests

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Before We Begin 🔗

While working through the TDD with Spring Boot↗ series, I kept seeing this red message in the console even when the tests passed.

OpenJDK 64-Bit Server VM warning

OpenJDK 64-Bit Server VM warning: Sharing is only supported for boot loader classes because bootstrap classpath has been appended

At first glance, it might look like an error. It's a

warning

, and it doesn't affect the functionality being tested. If seeing red text every time bothers you, this extra installment is for you.

You can also jump straight to the fix.

Why the Warning Appears 🔗

To understand this warning, we need to look at a feature of the Java Virtual Machine (JVM) called

Class Data Sharing (CDS)

What Is CDS? 🔗

Loading many classes every time a Java program starts takes time.

The JVM preprocesses information about commonly used classes and stores it in a

shared archive

The next time the program starts, it can read from that archive and start faster.

Why Do Tests Trigger the Warning? 🔗

Running

tests

changes the situation. Tools such as Mockito and JaCoCo, a test coverage tool, participate in the execution process.

As part of that process, they append entries to Java's bootstrap class path.

From the JVM's point of view, it goes something like this.

‘The current environment doesn't match the notes I prepared in the CDS archive.
Someone appended something.
I can't use those notes, so I'll read the classes myself.’

The JVM issues a warning about the restriction on sharing. It's effectively saying that

an optimization has been disabled

How to Suppress the Warning 🔗

You can safely ignore this warning.

If the red text bothers you, configure the JVM to disable CDS.

It will then skip sharing in the first place, so the warning goes away.

1. Configure IntelliJ IDEA for Individual Runs 🔗

To apply the change only when running from IntelliJ, follow these steps.

Open
Run > Edit Configurations...
from the top menu.
Select your JUnit test configuration on the left.
Enter -Xshare:off in
VM options
. If you don't see that field, open Modify options and enable Add VM options.

IntelliJ IDEA run configuration

2. Configure build.gradle for the Whole Project 🔗

Change the Gradle configuration to apply the setting across the project so the rest of your team also stops seeing the warning.

Add this configuration to the test task in build.gradle.

build.gradle

tasks.named('test') {
    useJUnitPlatform()
    
    // [추가] JVM에게 CDS 기능을 끄라고 지시합니다.
    jvmArgs '-Xshare:off'
}

Click the elephant icon in the upper-right corner to refresh Gradle, then rerun the tests.

Verify the Result 🔗

After applying the setting, run the tests again.

Test output after suppressing the warning

The OpenJDK 64-Bit Server VM warning message should disappear, leaving just the test results.

Conclusion 🔗

Today we looked at a JVM warning you may encounter when running tests.

Cause
Test tools modify the execution environment, and the JVM warns that the CDS optimization has been disabled.
Fix
Add -Xshare:off to turn off CDS explicitly and suppress the warning.

Thank you for reading.

References 🔗

Class Data Sharing (CDS)↗
Oracle's official explanation of CDS and how it works.
Gradle Test Task Configuration↗
Gradle's documentation on configuring JVM options for test tasks.

Fixing the ‘Sharing is only supported for boot loader classes’ Warning in Spring Boot Tests | PromleeBlog