Java Runtime errors

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Core environment and runtime library

Documented:

Top patterns

  1. A class that is not available on the classpath or the class loader is closed

    java.lang.ClassNotFoundException is one of the most frequent exceptions in Java. It is a runtime error, however, it does not signal a bug in your application logic, but rather an issue with the environment (build system, dependency management, runtime environment, etc.)

    According to the javadoc, it "is thrown when an application tries to load in a class through its string name". There are several different situations leading to this error and you have to examine the stack trace thoroughly to be able to decide how to solve this problem.

    The first thing to check is whether you get this checked exception directly, or wrapped in a java.class.NoClassDefFoundException. The latter happens when the class not found was resolved during compilation time, but not found during runtime, which is a bit different than the first case and it needs special care. If this is what happened to you, then you should read that article (coming soon).

    Next we ask what is the class that was not found. Fortunately, in most cases, you can see it in the message of the exception. If this class was specified by you (either directly in the code, or via a configuration file), double check that you did not misspell it. Some practical advices:

    • Make sure you use the fully qualified name of the class (e.g. org.postgresql.ds.PGSimpleDataSource)
    • When loading a nested class, make sure you use the proper name (joined with a $ sign, e.g. test.MyClass$InnerClass).
    • If you are unsure, you can check the name of the class in the code by using MyClass.class.getName()

    If none of the above helped then you have an issue with your classpath, which requires some time and experience to resolve. When the java.lang.ClassNotFoundException is related to a third party library, you might want to ask for help in their support forums. However, if you want to debug it yourself, let me equip you with some theoretical and practical knowledge.

    • To understand what is class loading in Java, start with this StackOverflow question or the Java Language Specification §12.2. Roughly speaking, the classes that your application use are located in jar files on the file system. The classpath is a list of jar files which are searched through when your application loads a class. The java.lang.ClassNotFoundException means that the class you wanted to load is not present in any of the jar files on the classpath.
    • The configuration of your classpath depends on your runtime environment (i.e. do you run the application from console, from IDE, web container, etc), you can find more information on the specific forums. However, if you can use breakpoints to stop at specific points in your application, stop at the first line of java.net.URLClassLoader.findClass and check the value of the field ucp.
    • You should have an idea on which jar file contains the class that could not be loaded, and look for that jar in the classpath. If it is not there, you have to add that jar to the classpath (precise details depend on the runtime environment).
    • If you find the jar you suspect to contain your class, check if it is true by opening the jar file (they are simple zip files actually) and look for the class definition in the directory structure in it (the directory structure must be identical to the package structure of the java code). At this point you should realize that the class is not present, otherwise the problem would have been found previously during debugging.
    • There are two reasons for class not being present in the jar: the first is that you have an incorrect version of the library you wanted to use. To fix this, you have to tweak your build system (maven, ant, sbt, gradle etc) by making sure that you configured the right version of the library as the dependency, and that you don’t have conflicting library versions. To find out how to do this, look for help on the forums of that build system.
    • The other reason of the class not being present is that it’s a bug in that library. If this is a third-party library, you might want to check the support forum and the release notes. If this library was written by you, then try to rebuild this library and check if the class you look for is present in the resulting jar file.
  2. ClassLoader.loadClass() has thrown a ClassNotFoundException
    No solution
  3. The class could not be found

    This error pattern looks similar to error pattern 88439.

    However, in this case, we are sure that this happens when sun.misc.URLClassPath.getResource(String name, boolean check) returns null. Practically every exception that happens in getResource is handled by returning null.


DescriptionException TypeEntry MethodWeb pages
NoClassDefFoundErrorEventDispatchThread.run4
NoClassDefFoundErrorEventDispatchThread.run2
NoClassDefFoundErrorEventDispatchThread.run1
NoClassDefFoundErrorEventDispatchThread.run1
NoClassDefFoundErrorEventDispatchThread.run1
NoClassDefFoundErrorEventDispatchThread.run1
NoClassDefFoundErrorEventDispatchThread.run1
Package nameClass namePatterns foundSolution coverageDescription coverage
java.awtEventQueue652
java.awtEventDispatchThread304
java.awtContainer295
java.awtComponent238
java.awtWindow80
java.awtToolkit64
java.awtAWTEventMulticaster59
java.awtFrame59
java.awtFont47
java.awtGraphicsEnvironment33
java.awtDialog31
java.awtLightweightDispatcher28
java.awtDesktop17
java.awtColor11
java.awtRobot10
java.awtComponent$FlipBufferStrategy10
java.awtDialog$19
java.awtComponent$BltBufferStrategy7
java.awtGridBagLayout7
java.awtToolkit$ToolkitEventMulticaster7
java.awtSplashScreen6
java.awtKeyboardFocusManager5
java.awtEventQueue$14
java.awtFocusTraversalPolicy4
java.awtMenuItem4
java.awtImage4
java.awtToolkit$24
java.awtAWTEvent3
java.awtButton3
java.awtGraphicsConfiguration3
java.awtDimension3
java.awtMenu3
java.awtBasicStroke3
java.awtDefaultKeyboardFocusManager3
java.awtCardLayout3
java.awtCanvas2
java.awtDialog$32
java.awtGraphicsDevice2
java.awtEventQueue$22
java.awtPanel2
java.awtScrollPane2
java.awtPopupMenu2
java.awtBorderLayout2
java.awtSystemTray2
java.awtAlphaComposite2
java.awtTrayIcon2
java.awtLabel2
java.awtSequencedEvent2
java.awtCursor1
java.awtCheckboxMenuItem1
Package nameClass nameMethod namePatterns foundSolution coverage
java.awtEventDispatchThreadpumpEvents149
java.awtEventDispatchThreadpumpEventsForFilter57
java.awtEventDispatchThreadpumpEventsForHierarchy38
java.awtEventDispatchThreadrun31
java.awtEventDispatchThreadpumpOneEventForHierarchy20
java.awtEventDispatchThreadpumpOneEventForFilters9
Package nameClass nameMethod nameException TypePatterns foundSolution coverage
java.awtEventDispatchThreadrunNullPointerException8
java.awtEventDispatchThreadrunNoClassDefFoundError7
java.awtEventDispatchThreadrunClassCastException3
java.awtEventDispatchThreadrunArrayIndexOutOfBoundsException3
java.awtEventDispatchThreadrunThrowable2
java.awtEventDispatchThreadrunInvocationException1
java.awtEventDispatchThreadrunAccessControlException1
java.awtEventDispatchThreadrunAbstractMethodError1
java.awtEventDispatchThreadrunIllegalMonitorStateException1
java.awtEventDispatchThreadrunUnsupportedMethodException1
java.awtEventDispatchThreadrunRuntimeException1
java.awtEventDispatchThreadrunSQLException1
java.awtEventDispatchThreadrunInvocationTargetException1