SpringBoot Executable JAR application class encounters performance issues when classpath URLs reference a host

We have several Spring boot applications running on windows servers. These applications start as an executable jar. Since the 3.2.0 release we noticed the startup times of these applications are a lot bigger. For example the startup time of one application has gone up from 16 seconds to 155 seconds. We also encountered performance issues at customers after running the application for a while. This only occurs when the max memory of the server is not very high and classes need to be reloaded. This can make the application unresponsive. The issues are resolved when we use the classic loader implementation.

I investigated the issue a bit further. The issue only seems to occur on windows. If I run the same spring boot application in a docker container the performance is good. The issue occurs on both OpenJDK 17 and 21. I am running on spring boot 3.5.0.

When the classes are loaded the threads seem to spend a lot of time in the following function:

"pool-2-thread-2" #61 [21940] prio=5 os_prio=0 cpu=406.25ms elapsed=4.92s tid=0x000002e84e6a2bf0 nid=21940 runnable  [0x000000ec0d1f8000]
   java.lang.Thread.State: RUNNABLE
        at java.panw.PanwHooks.NativeMethodEntry(Native Method)
        at java.panw.PanwHooks.MethodEntry1(Unknown Source)
        at java.net.URLStreamHandler.getHostAddress(java.base@21.0.6/URLStreamHandler.java)
        at java.net.URLStreamHandler.hostsEqual(java.base@21.0.6/URLStreamHandler.java:459)
        at java.net.URLStreamHandler.sameFile(java.base@21.0.6/URLStreamHandler.java:431)
        at java.net.URLStreamHandler.equals(java.base@21.0.6/URLStreamHandler.java:352)
        at java.net.URL.equals(java.base@21.0.6/URL.java:1144)
        at java.util.concurrent.ConcurrentHashMap.computeIfAbsent(java.base@21.0.6/ConcurrentHashMap.java:1721)
        at org.springframework.boot.loader.net.protocol.jar.JarFileUrlKey.get(JarFileUrlKey.java:49)
        at org.springframework.boot.loader.net.protocol.jar.UrlJarFiles$Cache.get(UrlJarFiles.java:157)
        at org.springframework.boot.loader.net.protocol.jar.UrlJarFiles.getCached(UrlJarFiles.java:81)
        at org.springframework.boot.loader.net.protocol.jar.JarUrlConnection.assertCachedJarFileHasEntry(JarUrlConnection.java:306)
        at org.springframework.boot.loader.net.protocol.jar.JarUrlConnection.connect(JarUrlConnection.java:287)
        at org.springframework.boot.loader.net.protocol.jar.JarUrlConnection.getJarFile(JarUrlConnection.java:99)
        at jdk.internal.loader.URLClassPath$Loader.getResource(java.base@21.0.6/URLClassPath.java:657)
        at jdk.internal.loader.URLClassPath.getResource(java.base@21.0.6/URLClassPath.java:316)
        at java.net.URLClassLoader$1.run(java.base@21.0.6/URLClassLoader.java:424)
        at java.net.URLClassLoader$1.run(java.base@21.0.6/URLClassLoader.java:421)
        at java.security.AccessController.executePrivileged(java.base@21.0.6/AccessController.java:809)
        at java.security.AccessController.doPrivileged(java.base@21.0.6/AccessController.java:714)
        at java.net.URLClassLoader.findClass(java.base@21.0.6/URLClassLoader.java:420)
        at java.lang.ClassLoader.loadClass(java.base@21.0.6/ClassLoader.java:593)
        - locked <0x000000061353b058> (a java.lang.Object)
        at org.springframework.boot.loader.net.protocol.jar.JarUrlClassLoader.loadClass(JarUrlClassLoader.java:107)
        at org.springframework.boot.loader.launch.LaunchedClassLoader.loadClass(LaunchedClassLoader.java:91)
        at java.lang.ClassLoader.loadClass(java.base@21.0.6/ClassLoader.java:526)

The issue can be easily reproduced when running an application with the configuration/ code below on windows 11 with openjdk 21. When I start this application, the startup time is about 20 seconds. When I start the application with the classic loader implementation the startup time is 3 seconds. In a docker container the startup times for both cases were about 2 seconds.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.example</groupId>
    <artifactId>spring-boot-demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.5.0</version>
        <relativePath/>
    </parent>

    <properties>
        <java.version>21</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <mainClass>com.example.demo.DemoApplication</mainClass>
                    <executable>true</executable>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

DemoApplication.java

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

The problem seems to be that the URL.equals() function in some cases (same URI values for both objects) is very slow on windows. I ran some performance tests with just that function. The performance was almost 15 times slower on windows than on linux.

Comment From: wilkinsona

Thanks for the report and analysis. It looks like we need to override equals(URL u1, URL u2) in org.springframework.boot.loader.net.protocol.jar.Handler. We already override hashCode(URL url) and doing something similar for equals should help here.

Comment From: bgoorden

Thanks for the response. That seems like a good solution.

Comment From: philwebb

java.net.URLStreamHandler already implements an equals method that delegates to the protected sameFile method that we override. At least it appears to on Linux. From the stack trace, it doesn't look like our overloaded version is being called.

Comment From: wilkinsona

Thanks, Phil. I stand corrected. So either the override isn't working for some reason or the URL doesn't have our custom handler.

Comment From: philwebb

JarFileUrlKey has a cache that attempts to save building strings for URLs. This seems to work for 90% or URLs and for most standard apps. If, however, a URL has a host then the URL.equals() method ends doing a DNS lookup.

I've not been able to replicate the issue on Windows, so my guess is there's something unique about the URLs on the classpath for these apps. Regardless, I think we should only use the cache if equals() is going to be cheap. If equals() is going to be expensive, we should just rebuild the string.