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
IllegalArgumentExceptionTimer.schedule29
Package nameClass namePatterns foundSolution coverageDescription coverage
java.utilArrayList175
java.utilArrays141
java.utilHashMap137
java.utilTreeMap72
java.utilCollections70
java.utilAbstractCollection70
java.utilLinkedList64
java.utilResourceBundle62
java.utilVector52
java.utilScanner51
java.utilHashSet45
java.utilTimerThread40
java.utilProperties40
java.utilHashtable37
java.utilServiceLoader$135
java.utilAbstractList34
java.utilTreeSet33
java.utilFormatter31
java.utilTimSort26
java.utilServiceLoader$LazyIterator25
java.utilTimer23
java.utilComparableTimSort22
java.utilCalendar21
java.utilAbstractList$Itr20
java.utilTimeZone19
java.utilLinkedHashMap18
java.utilAbstractMap17
java.utilDate15
java.utilFormatter$FormatSpecifier14
java.utilCollections$UnmodifiableCollection$114
java.utilServiceLoader14
java.utilGregorianCalendar13
java.utilCollections$UnmodifiableCollection12
java.utilCollections$UnmodifiableList11
java.utilLocale11
java.utilOptional11
java.utilStringTokenizer11
java.utilUUID10
java.utilArrayList$Itr10
java.utilWeakHashMap10
java.utilHashMap$HashIterator9
java.utilArrayList$SubList8
java.utilSubList8
java.utilHashMap$KeyIterator8
java.utilArrayDeque7
java.utilBitSet7
java.utilStack7
java.utilLinkedHashMap$LinkedHashIterator7
java.utilLinkedList$ListItr7
java.utilCollections$UnmodifiableMap7
Package nameClass nameMethod namePatterns foundSolution coverage
java.utilTimer<init>13
java.utilTimerschedule4
java.utilTimerscheduleAtFixedRate3
java.utilTimerscheduleImpl2
java.utilTimersched1
Package nameClass nameMethod nameException TypePatterns foundSolution coverage
java.utilTimerscheduleIllegalStateException2
java.utilTimerscheduleNullPointerException1
java.utilTimerscheduleIllegalArgumentException1