A NEW PLATFORM FOR A NEW ERA. Copyright 2014 Pivotal. All rights reserved.

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Leona Patrick
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1 A NEW PLATFORM FOR A NEW ERA 1

2 2 Java Memory Leaks in Modular Environments Mark Thomas November 2016

3 Introductions Mark Thomas Tomcat committer and PMC member Joined Tomcat community in 2003 Authored initial versions of: Tomcat s JreMemoryLeakPreventionListener Tomcat s leak detection on web application stop 3

4 Background Tomcat 7 / Servlet 3.0 presentation in 2009 An off-the-cuff remark PermGen out of memory errors are not the result of Tomcat bugs, they are application bugs Followed by a LOT of discussion Sat down with one of the attendees who had their app on their laptop Debugged the memory leak Root cause was a JRE bug And the JreMemoryLeakPreventionListener was born 4

5 Java Memory Management JVM allocates memory as objects are created Periodically, garbage collection is performed Identifies those objects still in use Frees memory allocated to all other objects Not concerned today about how this happens A memory leak occurs when a reference is unintentionally retained to an object making it ineligible for garbage collection Note: Classes have an object representation too 5

6 Class loaders An object retains a reference to the class of which it is an instance A class retains a reference to the class loader that loaded it A class loader retains a reference to all the classes it has loaded So it doesn t load the same class twice However 6

7 Class loaders A Java class is uniquely identified by Its name; and The class loader that loaded it You can have two versions of the same class at the same time if they have different class loaders Modular environments take advantage of this In OSGI different modules can depend on different version of the same library without conflict In Servlet containers, different web applications can depends on different versions of the same library 7

8 How things go wrong Requirements often exists to add, remove and replace modules dynamically The modules are typically each allocated a dedicated class loader When the module is removed / replaced garbage collection should remove Objects created by that module The classes associated with those objects The class loader that loaded those classes 8

9 How things go wrong If a reference is retained to an object that is an instance of a module loaded class The object retains a reference to its class The class retains a reference to the class loader The class loaded retains a reference to all the classes it has loaded And there you have a memory leak Bulk of memory leak consists of classes PermGen Metaspace Sometimes the retained object is the class loader 9

10 How to spot a leak Use a profiler YourKit give Apache committers a free license for their Apache work Unload a module May need to trigger explicit GC Look for a class loader that should have been GC d but hasn t been In Tomcat the state attribute helps Trace the GC roots for that class loader instance 10

11 Examples These all use JRE bugs to demonstrate them To help you avoid these specific bugs To help you avoid similar bugs in your own code Many of these went unfixed for a long time OpenJDK 9 team has been very responsive Guidance for correct RMI usage provided Nearly all remaining issues will be fixed for Java 9 The class comments for each test identify which Oracle Java versions are affected 11

12 Demonstration 12

13 Conclusions Things to be careful with Threads Class loader references Thread context class loader Exceptions Look for leaks before they cause problems 13

14 Questions 14

15 Thank You. 15

16 A NEW PLATFORM FOR A NEW ERA 16

Lesson 2 Dissecting Memory Problems

Lesson 2 Dissecting Memory Problems Poonam Parhar JVM Sustaining Engineer Oracle Agenda 1. Symptoms of Memory Problems 2. Causes of Memory Problems 3. OutOfMemoryError messages 3 Lesson 2-1 Symptoms of