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
NullPointerExceptionClass.forName163
NullPointerExceptionClass.forName9
NullPointerExceptionClass.forName7
NullPointerExceptionClass.forName3
NullPointerExceptionClass.forName3
NullPointerExceptionClass.forName2
NullPointerExceptionClass.forName2
NullPointerExceptionClass.forName2
NullPointerExceptionClass.forName2
NullPointerExceptionClass.forName1
NullPointerExceptionClass.forName1
NullPointerExceptionClass.forName1
Package nameClass namePatterns foundSolution coverageDescription coverage
java.langClassLoader2572
java.langClass2063
java.langThread1270
java.langString305
java.langThrowable209
java.langSystem163
java.langRuntime157
java.langStringBuilder111
java.langSecurityManager98
java.langStringBuffer56
java.langInteger55
java.langDouble47
java.langProcessBuilder47
java.langObject44
java.langAbstractStringBuilder42
java.langLong33
java.langException33
java.langThreadGroup31
java.langFloat29
java.langStringCoding21
java.langProcessImpl15
java.langClassLoader$NativeLibrary13
java.langByte12
java.langEnum12
java.langRuntimeException12
java.langUNIXProcess11
java.langThreadLocal11
java.langCharacter10
java.langNumberFormatException8
java.langClassNotFoundException8
java.langError7
java.langIterable6
java.langShort6
java.langNullPointerException5
java.langSecurityException4
java.langClassLoader$14
java.langNoClassDefFoundError4
java.langIllegalArgumentException3
java.langMath3
java.langBoolean3
java.langShutdown3
java.langClassCastException3
java.langLinkageError3
java.langProcessEnvironment2
java.langPackage2
java.langClassloader2
java.langStringCoding$StringDecoder2
java.langIllegalStateException2
java.langThreadLocal$ThreadLocalMap2
java.langNumber1
Package nameClass nameMethod namePatterns foundSolution coverage
java.langClassforName707
java.langClassnewInstance265
java.langClassprivateGetDeclaredMethods112
java.langClassgetDeclaredMethods98
java.langClassgetConstructor089
java.langClassprivateGetDeclaredConstructors72
java.langClassforName072
java.langClassgetDeclaredFields59
java.langClassgetMethod56
java.langClassgetMethods46
java.langClassgetConstructor36
java.langClassinitializeClass33
java.langClassnewInstance030
java.langClassprivateGetPublicMethods28
java.langClassgetDeclaredMethod28
java.langClassgetDeclaredConstructor25
java.langClassgetDeclaredConstructors23
java.langClassgetAnnotation22
java.langClassprivateGetDeclaredFields21
java.langClassgetDeclaredField19
java.langClassgetMethod019
java.langClassgetConstructors12
java.langClassgetResourceAsStream9
java.langClassinitAnnotationsIfNecessary9
java.langClasscheckMemberAccess9
java.langClassforName18
java.langClassgetSimpleName7
java.langClassgetAnnotations7
java.langClassinitialize7
java.langClassgetDeclaredMethods06
java.langClassgetDeclaredConstructors06
java.langClassnewInstanceImpl6
java.langClassgetDeclaredAnnotations6
java.langClassgetField6
java.langClassclassForName6
java.langClassisAnnotationPresent5
java.langClassannotationData5
java.langClassgetGenericInterfaces5
java.langClassgetCanonicalName5
java.langClassgetProtectionDomain5
java.langClassgetDeclaringClass5
java.langClassgetGenericSuperclass5
java.langClassgetClassLoader4
java.langClassgetEnclosingClass4
java.langClassgetField03
java.langClassgetFields3
java.langClassgetDexCacheType3
java.langClassgetSimpleBinaryName3
java.langClassasSubclass3
java.langClassgetDeclaredFields02
Package nameClass nameMethod nameException TypePatterns foundSolution coverage
java.langClassforNameClassNotFoundException247
java.langClassforNameNoClassDefFoundError156
java.langClassforNameUnsupportedClassVersionError43
java.langClassforNameUnsatisfiedLinkError23
java.langClassforNameClassFormatError19
java.langClassforNameIllegalStateException15
java.langClassforNameSecurityException14
java.langClassforNameExceptionInInitializerError14
java.langClassforNameVerifyError12
java.langClassforNameNullPointerException12
java.langClassforNameInternalError12
java.langClassforNameAccessControlException11
java.langClassforNameIncompatibleClassChangeError11
java.langClassforNamePluginException10
java.langClassforNameLinkageError7
java.langClassforNameNoSuchMethodError7
java.langClassforNameIOException5
java.langClassforNameOutOfMemoryError5
java.langClassforNameStackOverflowError4
java.langClassforNameIllegalAccessError3
java.langClassforNameRuntimeException3
java.langClassforNameNoSuchFieldError2
java.langClassforNameDriver2
java.langClassforNameInputSplitWithLocationInfo2
java.langClassforNameIllegalArgumentException2
java.langClassforNameMalformedObjectNameException2
java.langClassforNameClassNotFoundException2
java.langClassforNameNoInitialContextException2
java.langClassforNameOracleDriver2
java.langClassforNameServletException1
java.langClassforNameFileTailer1
java.langClassforNameResultObject1
java.langClassforNameError1
java.langClassforNameMappingException1
java.langClassforNameTableOutputFormat1
java.langClassforNameSparkException1
java.langClassforNameDownloadException1
java.langClassforNameYarnClientClusterScheduler1
java.langClassforNameFileNotFoundException1
java.langClassforNameExtendedClassNotFoundException1
java.langClassforNameBundleException1
java.langClassforNameUserDatabaseLoginModule1
java.langClassforNameStatus1
java.langClassforNameSecretException1
java.langClassforNameAdditionReduce1
java.langClassforNameExceptionInInitializerError1
java.langClassforNameClassNotFoundException1
java.langClassforNameTaskSetManager1
java.langClassforNameCPMImpl1
java.langClassforNameLIBLINEARStringOutcomeClassifierBuilder1