一、CDS 是什么？
类数据共享 （CDS） 是一项 JVM 功能，可帮助减少 Java 应用程序的启动时间和内存占用。从 JDK 12 开始，默认的 CDS 归档文件与 Oracle JDK 二进制文件一起预打包。笔者测试使用的 OpenJDK 64-Bit Server VM Zulu21.34+19-CA (build 21.0.3+9-LTS, mixed mode, sharing)它也是支持 CDS 的。
二、如何使用
2.1 训练
要使用它，您应该首先以分解形式对应用程序执行训练运行：
$ java -Djarmode=tools -jar my-app.jar extract --destination application
$ cd application
$ java -XX:ArchiveClassesAtExit=application.jsa -Dspring.context.exit=onRefresh -jar my-app.jar这将创建一个 application 目录并生成 application.jsa，只要应用程序未更新，就可以重复使用。
2.2 使用
要使用缓存，您需要在启动应用程序时添加一个额外的 -XX:SharedArchiveFile 参数：
$ java -XX:SharedArchiveFile=application.jsa -jar my-app.jar三、效果
为了测试 CDS，笔者使用 Spring Boot initializr 生成了一个 demo 项目。下面是 CDS 训练后的 application 目录的结构：
$ tree application
application
|-- application.jsa
|-- demo-0.0.1-SNAPSHOT.jar
`-- lib
|-- jackson-annotations-2.17.1.jar
|-- jackson-core-2.17.1.jar
|-- jackson-databind-2.17.1.jar
|-- jackson-datatype-jdk8-2.17.1.jar
|-- jackson-datatype-jsr310-2.17.1.jar
|-- jackson-module-parameter-names-2.17.1.jar
|-- jakarta.annotation-api-2.1.1.jar
|-- jul-to-slf4j-2.0.13.jar
|-- log4j-api-2.23.1.jar
|-- log4j-to-slf4j-2.23.1.jar
|-- logback-classic-1.5.6.jar
|-- logback-core-1.5.6.jar
|-- micrometer-commons-1.13.0.jar
|-- micrometer-observation-1.13.0.jar
|-- slf4j-api-2.0.13.jar
|-- snakeyaml-2.2.jar
|-- spring-aop-6.1.8.jar
|-- spring-beans-6.1.8.jar
|-- spring-boot-3.3.0.jar
|-- spring-boot-autoconfigure-3.3.0.jar
|-- spring-boot-jarmode-tools-3.3.0.jar
|-- spring-context-6.1.8.jar
|-- spring-core-6.1.8.jar
|-- spring-expression-6.1.8.jar
|-- spring-jcl-6.1.8.jar
|-- spring-web-6.1.8.jar
|-- spring-webmvc-6.1.8.jar
|-- tomcat-embed-core-10.1.24.jar
|-- tomcat-embed-el-10.1.24.jar
`-- tomcat-embed-websocket-10.1.24.jar
1 directory, 32 files3.1 直接运行 demo-0.0.1-SNAPSHOT.jar
$ java -jar demo-0.0.1-SNAPSHOT.jar
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v3.3.0)
2024-05-31T08:53:39.964+08:00 INFO 14832 --- [demo] [ main] com.example.demo.DemoApplication : Starting DemoApplication v0.0.1-SNAPSHOT using Java 21.0.3 with PID 14832 (D:\test\demo\target\demo-0.0.1-SNAPSHOT.jar started by L.cm in D:\test\demo\target)
2024-05-31T08:53:39.967+08:00 INFO 14832 --- [demo] [ main] com.example.demo.DemoApplication : No active profile set, falling back to 1 default profile: "default"
2024-05-31T08:53:40.893+08:00 INFO 14832 --- [demo] [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port 8888 (http)
2024-05-31T08:53:40.908+08:00 INFO 14832 --- [demo] [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2024-05-31T08:53:40.908+08:00 INFO 14832 --- [demo] [ main] o.apache.catalina.core.StandardEngine : Starting Servlet engine: [Apache Tomcat/10.1.24]
2024-05-31T08:53:40.948+08:00 INFO 14832 --- [demo] [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2024-05-31T08:53:40.949+08:00 INFO 14832 --- [demo] [ main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 921 ms
2024-05-31T08:53:41.257+08:00 INFO 14832 --- [demo] [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port 8888 (http) with context path '/'
2024-05-31T08:53:41.274+08:00 INFO 14832 --- [demo] [ main] com.example.demo.DemoApplication : Started DemoApplication in 1.702 seconds (process running for 2.143)我们可以在日志 Started DemoApplication in 1.702 seconds 看到启动耗时为 1.702 秒。
3.2 使用 CDS 运行
需要先 cd 到训练的 application 目录。
$ java -XX:SharedArchiveFile=application.jsa -jar demo-0.0.1-SNAPSHOT.jar
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v3.3.0)
2024-05-31T08:53:15.828+08:00 INFO 21444 --- [demo] [ main] com.example.demo.DemoApplication : Starting DemoApplication v0.0.1-SNAPSHOT using Java 21.0.3 with PID 21444 (D:\test\demo\target\application\demo-0.0.1-SNAPSHOT.jar started by L.cm in D:\test\demo\target\application)
2024-05-31T08:53:15.830+08:00 INFO 21444 --- [demo] [ main] com.example.demo.DemoApplication : No active profile set, falling back to 1 default profile: "default"
2024-05-31T08:53:16.244+08:00 INFO 21444 --- [demo] [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port 8888 (http)
2024-05-31T08:53:16.249+08:00 INFO 21444 --- [demo] [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2024-05-31T08:53:16.249+08:00 INFO 21444 --- [demo] [ main] o.apache.catalina.core.StandardEngine : Starting Servlet engine: [Apache Tomcat/10.1.24]
2024-05-31T08:53:16.272+08:00 INFO 21444 --- [demo] [ main] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2024-05-31T08:53:16.273+08:00 INFO 21444 --- [demo] [ main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 419 ms
2024-05-31T08:53:16.425+08:00 INFO 21444 --- [demo] [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat started on port 8888 (http) with context path '/'
2024-05-31T08:53:16.431+08:00 INFO 21444 --- [demo] [ main] com.example.demo.DemoApplication : Started DemoApplication in 0.722 seconds (process running for 0.885)我们可以在日志 Started DemoApplication in 0.722 seconds 看到启动耗时比直接启动少了将近 1 秒。效果还是非常明显的。
四、总结
CDS、CRaC 和 GraalVM，这三种技术都有助于提高Java程序的启动速度，但它们的应用场景和优化方式有所不同。CDS 通过共享类数据来加速启动，CRaC 通过运行时优化来提升性能，而 GraalVM 则通过 AOT 编译来实现快速启动和高效运行。作为开发者，我们可以根据具体需求选择合适的技术来优化 Java 程序的启动时间。
