JAX London Presentation 2014

1. JAVA CLASSLOADERS
GO SIMON MAPLE
@SJMAPLE

2. ABOUT ME
SIMON MAPLE
@SJMAPLE
www.zeroturnaround.com support@zeroturnaround.com 1 (857) 277-1199 545 Boylston St., 4th flr. Boston, MA, USA, 02116

3. BACKGROUND: JREBEL
• Hooked into class loading on the JVM level
• Integrated with the class loading mechanism in more than 20
different servers
• Solved hundreds of issues connected to class loading
• Learned a lot more about class loaders than we wanted to ☺
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4. PLAN
• Basics
• What is class loading?
• How was it meant to work?
• Problems and solutions
• How do class loaders leak?
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5. BASICS
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6. JAVA.LANG.CLASSLOADER
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7. EXAMPLE 0
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8. EXAMPLE 0
Causes a call to
A.class.getClassLoader().loadClass(“B”);
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9. CL HIERARCHY
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10. INTENTIONS
• Class loaders have a parent class loader
• The parent is usually consulted first
• However in a Java EE web module local classes are searched first
• Each WAR module of EAR gets a classloader
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11. PROBLEMS
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12. NOT ENOUGH CLASSES
• ClassNotFoundException
•NoClassDefFoundError
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13. TROUBLESHOOTING
• IDE class lookup (Ctrl+Shift+T in Eclipse)
• find *.jar -exec jar -tf '{}' ; | grep MyClass
• URLClassLoader.getUrls()
• Container specific logs
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14. ALIEN CLASSES
• IncompatibleClassChangeError
• AbstractMethodError
•NoSuch(Method|Field)FoundError
• ClassCastException, IllegalAccessError
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15. TROUBLESHOOTING
• -verbose:class
• ClassLoader.getResource()
• javap -private MyClass
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16. MORE CLASSES
THAN YOU NEED
• LinkageError (class loading constraints violated)
• ClassCastException, IllegalAccessError
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17. TROUBLESHOOTING
• -verbose:class
• ClassLoader.getResource()
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18. new Util3() Factory3.instanceUntyped();
Util3 u = (Util3) Factory3.instanceUntyped();
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19. CLASSLOADER LEAKS
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20. CLASSLOADER LEAKS
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21. TAKE AWAY
• Validate assumptions
• Leaking ClassLoader is easy
• Isolation works for processes
• JREBEL rocks!
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22. ELSE
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