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Java Classloader

April 12, 2024

The Java Class Loader, part of the Java Runtime Environment, dynamically loads Java classes into the Java Virtual Machine.[1] Usually classes are only loaded on demand. The virtual machine will only load the class files required for executing the program.[2] The Java run time system does not need to know about files and file systems as this is delegated to the class loader.

A software library is a collection of related object code. In the Java language, libraries are typically packaged in JAR files. Libraries can contain objects of different types. The most important type of object contained in a Jar file is a Java class. A class can be thought of as a named unit of code. The class loader is responsible for locating libraries, reading their contents, and loading the classes contained within the libraries. This loading is typically done "on demand", in that it does not occur until the class is called by the program. A class with a given name can only be loaded once by a given class loader.

Each Java class must be loaded by a class loader.[3][4] Furthermore, Java programs may make use of external libraries (that is, libraries written and provided by someone other than the author of the program) or they may be composed, at least in part, of a number of libraries.

When the JVM is started, three class loaders are used:[5][6][2]

  1. Bootstrap class loader
  2. Extensions class loader
  3. System class loader

The bootstrap class loader loads the core Java libraries[fn 1] located in the <JAVA_HOME>/jre/lib (or <JAVA_HOME>/jmods> for Java 9 and above) directory. This class loader, which is part of the core JVM, is written in native code. The bootstrap class loader is not associated with any ClassLoader object.[2] For instance, StringBuilder.class.getClassLoader() returns null.[2]

The extensions class loader loads the code in the extensions directories (<JAVA_HOME>/jre/lib/ext,[5] or any other directory specified by the java.ext.dirs system property).

The system class loader loads code found on java.class.path, which maps to the CLASSPATH environment variable.

User-defined class loaders edit

The Java class loader is written in Java. It is therefore possible to create your own class loader without understanding the finer details of the Java Virtual Machine. Apart from the Bootstrap class loader, every Java class loader has a parent class loader.[7] The parent class loader is defined when a new class loader is instantiated or set to the virtual machine's system default class loader.

This makes it possible (for example):

  • to load or unload classes at runtime (for example to load libraries dynamically at runtime, even from an HTTP resource). This is an important feature for:
    • implementing scripting languages, such as Jython
    • using bean builders
    • allowing user-defined extensibility
    • allowing multiple namespaces to communicate. This is one of the foundations of CORBA / RMI protocols for example.
  • to change the way the bytecode is loaded (for example, it is possible to use encrypted Java class bytecode[8]).
  • to modify the loaded bytecode (for example, for load-time weaving of aspects when using aspect-oriented programming).

Class loaders in Jakarta EE edit

Jakarta EE (formerly Java EE and J2EE) application servers typically load classes from a deployed WAR or EAR archive by a tree of class loaders, isolating the application from other applications, but sharing classes between deployed modules. So-called "servlet containers" are typically implemented in terms of multiple class loaders.[4][9]

JAR hell edit

JAR hell is a term similar to DLL hell used to describe all the various ways in which the classloading process can end up not working.[10] Three ways JAR hell can occur are:

  • Accidental presence of two different versions of a library installed on a system. This will not be considered an error by the system. Rather, the system will load classes from one or the other library. Adding the new library to the list of available libraries instead of replacing it may result in the application still behaving as though the old library is in use, which it may well be.
  • Multiple libraries or applications require different versions of library foo. If versions of library foo use the same class names, there is no way to load the versions of library foo with the same class loader.
  • The most complex JAR hell problems arise in circumstances that take advantage of the full complexity of the classloading system. A Java program is not required to use only a single "flat" class loader, but instead may be composed of several (potentially very many) nested, cooperating class loaders. Classes loaded by different class loaders may interact in complex ways not fully comprehended by a developer, leading to errors or bugs that are difficult to analyze, explain, and resolve.[11]

The OSGi Alliance specified (starting as JSR 8 in 1998) a modularity framework that aims to solve JAR hell for current and future VMs in ME, SE, and EE that is widely adopted. Using metadata in the JAR manifest, JAR files (called bundles) are wired on a per-package basis. Bundles can export packages, import packages and keep packages private, providing the basic constructs of modularity and versioned dependency management.

To remedy the JAR hell problems, a Java Community Process — JSR 277 was initiated in 2005. The resolution — Java Platform Module System — intended to introduce a new distribution format, a modules versioning scheme, and a common modules repository (similar in purpose to Microsoft .NET's Global Assembly Cache). In December 2008, Sun announced that JSR 277 was put on hold.[12] The Java Module System was later rebooted as "project Jigsaw"[13] which was included in Java 9. Released in 2017, it includes support for modular software, termed "Java Platform Module System", which is controlled on source level with module-info.java files. It follows a different philosophy from the OSGi architecture that aims at providing modularity for the Java Runtime Environment in a backwards-compatible way that uses the default mechanism of loading classes that the JRE provides. However, since it does not offer the ability for controlled co-existence of libraries with different versions, it is not suited for tackling the JAR hell problem.[14]

See also edit

Footnotes edit

  1. ^ These libraries are stored in Jar files called rt.jar, core.jar, server.jar, etc.

References edit

  1. ^ Mcmanis, Chuck (October 1, 1996). "The basics of Java class loaders". JavaWorld. Retrieved 2020-07-13.
  2. ^ a b c d Horstmann 2022, §10.1.1 The Class-Loading Process.
  3. ^ Horstmann 2022, §8.2.5 Writing Byte Codes to Memory.
  4. ^ a b Christudas, Binildas (January 26, 2005). . onjava.com. Archived from the original on 2018-05-10.
  5. ^ a b "Understanding Extension Class Loading". The Java Tutorials. docs.oracle.com. Retrieved 2020-07-13.
  6. ^ Sosnoski, Dennis (April 29, 2003). "Classes and class loading". IBM DeveloperWorks. Retrieved 2008-01-26.
  7. ^ Horstmann 2022, 10.1.2 The Class Loader Hierarchy.
  8. ^ Roubtsov, Vladimir (May 9, 2003). "Cracking Java byte-code encryption". JavaWorld. Retrieved 2020-07-13.
  9. ^ deBoer, Tim; Karasiuk, Gary (August 21, 2002). "J2EE Class Loading Demystified". IBM DeveloperWorks. Retrieved 2008-01-26.
  10. ^ . Archived from the original on 2013-06-01.
  11. ^ "Taxonomy of class loader problems with Jakarta Commons Logging".
  12. ^ Mark Reinhold (September 20, 2010). . Oracle Corporation. Archived from the original on 2015-12-08.
  13. ^ "Project Jigsaw". Oracle Corporation. Retrieved 2015-11-29.
  14. ^ Bartlett, Neil; Hackbarth, Kai (2016-09-22). "Java 9, OSGi and the Future of Modularity (Part 1)". InfoQ.

External links edit

  • Chuck McManis, "The basics of Java class loaders", 1996
  • Brandon E. Taylor, "Java Class Loading: The Basics 2020-11-09 at the Wayback Machine", 2003
  • Horstmann, Cay (April 15, 2022). Core Java. Oracle Press Java. ISBN 0-13-787107-4.
  • Jeff Hanson, "Take Control of Class Loading in Java 2020-12-04 at the Wayback Machine", 2006-06-01
  • Andreas Schaefer, "", 2003-11-12
  • Sheng Liang and Gilad Bracha, "Dynamic class loading in the Java virtual machine", In Proceedings of the 13th ACM Conference on Object-Oriented Programming, Systems, Languages, and Applications (OOPSLA'98), ACM SIGPLAN Notices, vol. 33, no. 10, ACM Press, 1998, pp. 36–44 doi:10.1145/286936.286945
  • Jeremy Whitlock, "", May 2005
  • Christoph G. Jung, "", Java Specialist Newsletter, 2001-06-07
  • Don Schwarz, "", 2005-04-13

April 12, 2024
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The Java Class Loader part of the Java Runtime Environment dynamically loads Java classes into the Java Virtual Machine 1 Usually classes are only loaded on demand The virtual machine will only load the class files required for executing the program 2 The Java run time system does not need to know about files and file systems as this is delegated to the class loader A software library is a collection of related object code In the Java language libraries are typically packaged in JAR files Libraries can contain objects of different types The most important type of object contained in a Jar file is a Java class A class can be thought of as a named unit of code The class loader is responsible for locating libraries reading their contents and loading the classes contained within the libraries This loading is typically done on demand in that it does not occur until the class is called by the program A class with a given name can only be loaded once by a given class loader Each Java class must be loaded by a class loader 3 4 Furthermore Java programs may make use of external libraries that is libraries written and provided by someone other than the author of the program or they may be composed at least in part of a number of libraries When the JVM is started three class loaders are used 5 6 2 Bootstrap class loader Extensions class loader System class loaderThe bootstrap class loader loads the core Java libraries fn 1 located in the lt JAVA HOME gt jre lib or lt JAVA HOME gt jmods gt for Java 9 and above directory This class loader which is part of the core JVM is written in native code The bootstrap class loader is not associated with any span class n ClassLoader span object 2 For instance span class n StringBuilder span span class p span span class na class span span class p span span class na getClassLoader span span class p span returns span class kc null span 2 The extensions class loader loads the code in the extensions directories lt JAVA HOME gt jre lib ext 5 or any other directory specified by the java ext dirs system property The system class loader loads code found on java class path which maps to the a href Classpath Java html class mw redirect title Classpath Java CLASSPATH a environment variable Contents 1 User defined class loaders 2 Class loaders in Jakarta EE 3 JAR hell 4 See also 5 Footnotes 6 References 7 External linksUser defined class loaders editThe Java class loader is written in Java It is therefore possible to create your own class loader without understanding the finer details of the Java Virtual Machine Apart from the Bootstrap class loader every Java class loader has a parent class loader 7 The parent class loader is defined when a new class loader is instantiated or set to the virtual machine s system default class loader This makes it possible for example to load or unload classes at runtime for example to load libraries dynamically at runtime even from an HTTP resource This is an important feature for implementing scripting languages such as Jython using bean builders allowing user defined extensibility allowing multiple namespaces to communicate This is one of the foundations of CORBA RMI protocols for example to change the way the bytecode is loaded for example it is possible to use encrypted Java class bytecode 8 to modify the loaded bytecode for example for load time weaving of aspects when using aspect oriented programming Class loaders in Jakarta EE editJakarta EE formerly Java EE and J2EE application servers typically load classes from a deployed WAR or EAR archive by a tree of class loaders isolating the application from other applications but sharing classes between deployed modules So called servlet containers are typically implemented in terms of multiple class loaders 4 9 JAR hell editJAR hell is a term similar to DLL hell used to describe all the various ways in which the classloading process can end up not working 10 Three ways JAR hell can occur are Accidental presence of two different versions of a library installed on a system This will not be considered an error by the system Rather the system will load classes from one or the other library Adding the new library to the list of available libraries instead of replacing it may result in the application still behaving as though the old library is in use which it may well be Multiple libraries or applications require different versions of library foo If versions of library foo use the same class names there is no way to load the versions of library foo with the same class loader The most complex JAR hell problems arise in circumstances that take advantage of the full complexity of the classloading system A Java program is not required to use only a single flat class loader but instead may be composed of several potentially very many nested cooperating class loaders Classes loaded by different class loaders may interact in complex ways not fully comprehended by a developer leading to errors or bugs that are difficult to analyze explain and resolve 11 The OSGi Alliance specified starting as JSR 8 in 1998 a modularity framework that aims to solve JAR hell for current and future VMs in ME SE and EE that is widely adopted Using metadata in the JAR manifest JAR files called bundles are wired on a per package basis Bundles can export packages import packages and keep packages private providing the basic constructs of modularity and versioned dependency management To remedy the JAR hell problems a Java Community Process JSR 277 was initiated in 2005 The resolution Java Platform Module System intended to introduce a new distribution format a modules versioning scheme and a common modules repository similar in purpose to Microsoft NET s Global Assembly Cache In December 2008 Sun announced that JSR 277 was put on hold 12 The Java Module System was later rebooted as project Jigsaw 13 which was included in Java 9 Released in 2017 it includes support for modular software termed Java Platform Module System which is controlled on source level with module info java files It follows a different philosophy from the OSGi architecture that aims at providing modularity for the Java Runtime Environment in a backwards compatible way that uses the default mechanism of loading classes that the JRE provides However since it does not offer the ability for controlled co existence of libraries with different versions it is not suited for tackling the JAR hell problem 14 See also editLoader computing Dynamic loading DLL hell OSGi Classpath Java Java Platform Module SystemFootnotes edit These libraries are stored in Jar files called rt jar core jar server jar etc References edit Mcmanis Chuck October 1 1996 The basics of Java class loaders JavaWorld Retrieved 2020 07 13 a b c d Horstmann 2022 10 1 1 The Class Loading Process Horstmann 2022 8 2 5 Writing Byte Codes to Memory a b Christudas Binildas January 26 2005 Internals of Java Class Loading onjava com Archived from the original on 2018 05 10 a b Understanding Extension Class Loading The Java Tutorials docs oracle com Retrieved 2020 07 13 Sosnoski Dennis April 29 2003 Classes and class loading IBM DeveloperWorks Retrieved 2008 01 26 Horstmann 2022 10 1 2 The Class Loader Hierarchy Roubtsov Vladimir May 9 2003 Cracking Java byte code encryption JavaWorld Retrieved 2020 07 13 deBoer Tim Karasiuk Gary August 21 2002 J2EE Class Loading Demystified IBM DeveloperWorks Retrieved 2008 01 26 Depot Apache Incubator Archived from the original on 2013 06 01 Taxonomy of class loader problems with Jakarta Commons Logging Mark Reinhold September 20 2010 Project Jigsaw Oracle Corporation Archived from the original on 2015 12 08 Project Jigsaw Oracle Corporation Retrieved 2015 11 29 Bartlett Neil Hackbarth Kai 2016 09 22 Java 9 OSGi and the Future of Modularity Part 1 InfoQ External links editChuck McManis The basics of Java class loaders 1996 Brandon E Taylor Java Class Loading The Basics Archived 2020 11 09 at the Wayback Machine 2003 Horstmann Cay April 15 2022 Core Java Oracle Press Java ISBN 0 13 787107 4 Jeff Hanson Take Control of Class Loading in Java Archived 2020 12 04 at the Wayback Machine 2006 06 01 Andreas Schaefer Inside Class Loaders 2003 11 12 Sheng Liang and Gilad Bracha Dynamic class loading in the Java virtual machine In Proceedings of the 13th ACM Conference on Object Oriented Programming Systems Languages and Applications OOPSLA 98 ACM SIGPLAN Notices vol 33 no 10 ACM Press 1998 pp 36 44 doi 10 1145 286936 286945 Jeremy Whitlock Real World Use For Custom ClassLoaders May 2005 Christoph G Jung Classloaders Revisited Hotdeploy Java Specialist Newsletter 2001 06 07 Don Schwarz Managing Component Dependencies Using ClassLoaders 2005 04 13 Retrieved from https en wikipedia org w index php title Java Classloader amp oldid 1216883953, wikipedia, wiki, book, books, library,

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