Join the war on Classloader leaks

Memory issues can lead to errors like OutOfMemoryError, affecting Java applications. Learn about PermGen space, Metaspace, and JVM memory management to prevent leaks and optimize performance. Explore solutions and examples to tackle these challenges efficiently.

• Java
• Memory Management
• Classloader Leaks
• JVM Memory
• OutOfMemoryError

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Uploaded on Feb 23, 2025 | 0 Views

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1. Join the war on Classloader leaks Mattias Jiderhamn Mattias Jiderhamn java.jiderhamn.se

2. :-( java.lang.OutOfMemoryError: PermGen space Metaspace Mattias Jiderhamn java.jiderhamn.se

3. Local dev env java.lang. OutOfMemoryError: Metaspace Mattias Jiderhamn java.jiderhamn.se

4. Continuous Deploy java.lang. OutOfMemoryError: Metaspace .war Mattias Jiderhamn java.jiderhamn.se

5. Agenda What does it mean? Why does it happen? Where does it leak? How to avoid? OSS examples Training! Mattias Jiderhamn java.jiderhamn.se

6. What? Mattias Jiderhamn java.jiderhamn.se

7. JVM memory PermGen / Metaspace Stack Heap Mattias Jiderhamn java.jiderhamn.se

8. JVM memory Per thread Local variables and method parameters Stack Mattias Jiderhamn java.jiderhamn.se

9. JVM memory Object instances Heap Young generation Eden space Old generation / tenured space Survivor space Mattias Jiderhamn java.jiderhamn.se

10. JVM memory Permanent Generation PermGen / Metaspace java.lang.Class instances etc Named before JEE and class unloading Renamed Metaspace in Java 8 Aka Method area Mattias Jiderhamn java.jiderhamn.se

11. What? java.lang.OutOfMemoryError: PermGen space / Metaspace = Too many classes are loaded Mattias Jiderhamn java.jiderhamn.se

12. Why? Mattias Jiderhamn java.jiderhamn.se

13. Reason for OOME java.lang.OutOfMemoryError: PermGen space / Metaspace 1. Your application is too large -XX:MaxPermSize=256M Java 8: Metaspace auto increase by default 2. java.lang.Class instances could not be garbage collected after redeploy Mattias Jiderhamn java.jiderhamn.se

14. Reference types Strong (i.e. normal) reference Never GC:ed if reachable Soft reference GC:ed before OutOfMemoryError Weak reference (WeakHashMap) GC:ed when no Strong or Soft refs Phantom reference won t prevent GC Mattias Jiderhamn java.jiderhamn.se

15. Example WeakHashMap Map m = new WeakHashMap(); Foo myFoo = new Foo(); m.put(myFoo, bar ); myFoo = null; Foo Mattias Jiderhamn java.jiderhamn.se

16. GC reachability GC roots Mattias Jiderhamn java.jiderhamn.se

17. Redeploy Application Server ClassLoader ClassLoader ClassLoader app.war app.war app.war Mattias Jiderhamn java.jiderhamn.se

18. ClassLoader refs ClassLoader Class Class Class Instance Mattias Jiderhamn java.jiderhamn.se

19. How leaks happen Application Server ClassLoader Class Class Class Instance GC root Mattias Jiderhamn java.jiderhamn.se

20. Where? Mattias Jiderhamn java.jiderhamn.se

21. Application Server Application Server bugs Logging frameworks Apache Commons Logging Unless LogFactory.release() Log4j - some configs Unless LogManager.shutdown() java.util.logging custom Level Bean Validation API (JSR 303) Unified Expression Language (javax.el) ? Mattias Jiderhamn java.jiderhamn.se

22. GC roots Class loaded by system ClassLoader static field in JDK classes (java.* etc) Live thread Stack local vars, method params java.lang.Thread instance Object held as synchronization monitor JNI references JVM specials Mattias Jiderhamn java.jiderhamn.se

23. System classes java.sql.DriverManager Bean introspection cache, shutdown hooks, custom default Authenticator, custom security Provider, custom MBeans, custom ThreadGroup, custom property editor, Reference tocontextClassLoader of first caller Mattias Jiderhamn java.jiderhamn.se

24. DriverManager Mattias Jiderhamn java.jiderhamn.se

25. DriverManager JRE Application Server DriverManager 1) ClassLoader com.mysql.jdbc.Driver app.war mysql-jdbc.jar or 2) DriverManager.deregisterDriver( ) Mattias Jiderhamn java.jiderhamn.se

26. Context shutdown public class MyCleanupListener implements javax.servlet.ServletContextListener { ... /** Called when application is undeployed */ public void contextDestroyed( ServletContextEvent servletContextEvent) { DriverManager.deregisterDriver( ); } } Mattias Jiderhamn java.jiderhamn.se

27. Threads Thread stack Local variables Method parameters MyThread extends Thread MyRunnable implements Runnable contextClassLoader + AccessControlContext inherited Example HTTP 1.1 Keep-Alive-Timer Mattias Jiderhamn java.jiderhamn.se

28. Context shutdown public class MyCleanupListener implements javax.servlet.ServletContextListener { ... /** Called when application is undeployed */ public void contextDestroyed( ServletContextEvent servletContextEvent) { DriverManager.deregisterDriver( ); // Stop threads here! } } Mattias Jiderhamn java.jiderhamn.se

29. Stopping threads public class MyThread extends Thread { public void run() { while(true) { // Bad idea! // Do something } } } Mattias Jiderhamn java.jiderhamn.se

30. Stopping threads public class MyThread extends Thread { private boolean running = true; private volatile boolean running = true; public void run() { while(running) { // Until stopped // Do something } } public void shutdown() { running = false; } Heinz Kabutz / Java Specialists - The Law of the Blind Spot } Mattias Jiderhamn java.jiderhamn.se

31. Threads Thread stack Local variables Method parameters MyThread extends Thread MyRunnable implements Runnable contextClassLoader + AccessControlContext inherited Example HTTP 1.1 Keep-Alive-Timer ThreadLocal Mattias Jiderhamn java.jiderhamn.se

32. ThreadLocal X X ThreadLocal WeakHashMap<Thread, ?> Foo Thread Mattias Jiderhamn java.jiderhamn.se

33. ThreadLocal ThreadLocal JavaDoc: Each thread holds an implicit reference to its copy of a thread- local variable as long as the thread is alive and the ThreadLocal instance is accessible; Mattias Jiderhamn java.jiderhamn.se

34. ThreadLocal Thread ThreadLocalMap Entry put() ThreadLocal Foo set() Mattias Jiderhamn java.jiderhamn.se

35. (?) ThreadLocal Pooled threads: Threads may outlive ClassLoader ThreadLocal ThreadGlobal! Mattias Jiderhamn java.jiderhamn.se

36. ThreadLocal Thread ThreadLocalMap Entry ThreadLocal Foo static Class ClassLoader Class Mattias Jiderhamn java.jiderhamn.se

37. ThreadLocal Thread ThreadLocalMap Entry Stale entry ThreadLocal Foo ClassLoader Class Mattias Jiderhamn java.jiderhamn.se

38. ThreadLocal ThreadLocalMap JavaDoc: However, since reference queues are not used, stale entries are guaranteed to be removed only when the table starts running out of space Mattias Jiderhamn java.jiderhamn.se

39. ThreadLocal Custom value (incl references) static ThreadLocal = leak otherwise = unpredicted GC Custom ThreadLocal = no leak Mattias Jiderhamn java.jiderhamn.se

40. ThreadLocal Always clear your ThreadLocals! try { myThreadLocal.set(foo); } finally { myThreadLocal.remove(); } Mattias Jiderhamn java.jiderhamn.se

41. Known offenders Apache ActiveMQ Apache Axis Apache Batik Apache Commons Pool / DBCP Apache CXF AWT Toolkit Bean Validation API / JSR 303 CGLIB (Hibernate / Spring / JBoss / Apache Geronimo) dom4j EclipseLink GeoTools Google Guice Groovy GWT / javax.imageio Hessian iCal4J Infinispan IntrospectionUtils JarURLConnection Java Advanced Imaging (JAI) Javassist Java Cryptography Architecture (JCA) Java Server Faces 2 javax.security.auth.Policy/login.Configuration JGroups Logback JAXB Mozilla Rhino MVEL OpenOffice Java Uno RunTime (JURT) Oracle JDBC RMI Serialization (<= JDK 1.4.2) Spring framework Unified Expression Language / javax.el URLConnection + HTTP 1.1 XML parsing (DocumentBuilderFactory) Mattias Jiderhamn java.jiderhamn.se

42. How? Mattias Jiderhamn java.jiderhamn.se

43. Leak Prevention Lib Application server independent Version 2 core module is pure Java SE! Covers a lot more than Tomcat System class references avoided/cleared Threads are stopped ThreadLocals are cleared Known offenders handled Logs warnings Apache 2 licensed You may modify and redistribute Mattias Jiderhamn java.jiderhamn.se

44. Leak Prevention Lib Zero runtime dependencies No required config Maven pom.xml (Servlet 3.0+): <dependency> <groupId>se.jiderhamn.classloader-leak-prevention</groupId> <artifactId>classloader-leak-prevention-servlet3</artifactId> <version>2.0.1</version> </dependency> Mattias Jiderhamn java.jiderhamn.se

45. Tomcat Cleanup Thread Thread get() set() remove() remove() ThreadLocalMap Unsafe! Removed in 6.0.27 Bugzilla #48895 Tomcat 7.0.6+ renews the thread pool whenever an application is redeployed. Disposable threads for lifecycle events. Bugzilla #49159 Mattias Jiderhamn java.jiderhamn.se

46. Leak Prevention Lib Cleanup Thread Thread set(null) ThreadLocalMap Entry Stale Foo ThreadLocal Mattias Jiderhamn java.jiderhamn.se

47. Open Source examples Mattias Jiderhamn java.jiderhamn.se

48. Bean Validation API Version 1.0.0.GA javax.validation.Validation Application Server validation-api-1.0.0.GA.jar app.war hibernate-validator.jar Mattias Jiderhamn java.jiderhamn.se

49. javax.el API javax.el.BeanELResolver Mattias Jiderhamn java.jiderhamn.se

50. Apache CXF org.apache.cxf.transport.http.CXFAuthenticator CXF-5442 Mattias Jiderhamn java.jiderhamn.se

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