WebLogic类加载过程简述

WebLogic类加载过程王明杰20061012 19:28-21:59英文水平有限，翻译的肯定有不对的地方，欢迎指正。

希望能够各位同事在使用weblogic的过程中带来帮助。

小注：2006-10-12 下午，在weblogic中部署一个启动类，部署了近2个小时，weblogic总是提示“找不到类的异常”。

心情很是郁闷。

“大事问老婆，小事问goole”，一个字“搜”。

终于找到了WebLogic的官方文档，摘录下来，进行了翻译，终于对WebLogic的类的加载过程有了比较系统的了解。

希望该文档能够帮助和我有同样疑惑的人走出困境，共同进步！！！V ersion 0.1msn:wangmingjie_2002@官方原版（19：30开始）20061012开始WebLogic Server Application ClassloadingThe following sections provide an overview of Java classloaders, followed by details about WebLogic Server J2EE application classloading.下面的部分概况的介绍了java的类装载器，接着详细的介绍了WebLogicJ2EE应用服务器（WebLogic Server J2EE application ）的类装载过程。

▪Java Classloader Overview java类转载器综述▪WebLogic Server Application Classloader Overview WebLogic 应用服务器类装载器介绍。

▪Resolving Class References Between Modules and ApplicationsJava Classloader OverviewClassloaders are a fundamental module of the Java language. A classloader is a part of the Java virtual machine (JVM) that loads classes into memory;a classloader is responsible for finding and loading class files at run time. Every successful Java programmer needs to understand classloaders and their behavior. This section provides an overview of Java classloaders.类装载器是java语言的一个基本模块，类装载器是java虚拟机的一部分，把相应的类装载到内存中，类装载器负责在运行期间找到类并载入类。

每个成功的java程序员都需要理解java类装载器及其行为。

本部分综述了java类装载器。

Java Classloader Hierarchy java类装载器的层次Classloaders contain a hierarchy with parent classloaders and child classloaders. The relationship between parent and child classloaders is analogous to the object relationship of super classes and subclasses. The bootstrap classloader is the root of the Java classloader hierarchy. The Java virtual machine (JVM) creates the bootstrap classloader, which loads the Java development kit (JDK) internal classes and java.* packages included in the JVM. (For example, the bootstrap classloader loads ng.String.)类装载器有层次，包含了父装载器和子装载器。

父装载器和子装载器直接的关系和java中的父类和子类的关系是相似的。

启动类装载器是根类装载器，它将jdk 的内部类和包含在java.*包中的类装载到java虚拟机。

（例如：启动类装载器载入ng.String类。

）The extensions classloader is a child of the bootstrap classloader. The extensions classloader loads any JAR files placed in the extensions directory of the JDK. This is a convenient means to extending the JDK without adding entries to the classpath. However, anything in the extensions directory must be self-contained and can only refer to classes in the extensions directory or JDK classes.（C:\j2sdk1.4.2_08\jre\lib\ext）扩这的类装载器是启动类装载器的子类装载器。

扩展类装载器会自动加载放在jdk扩展文件夹中的jar包。

这很方便，意味着扩展JDK不需要向classpath中增加信息。

然而扩展文件夹中的任何包都必须独立，只能参照扩展文件夹中的类或者JDK的类。

（最后这句翻译的不好。

）The system classpath classloader extends the JDK extensions classloader. The system classpath classloader loads the classes from the classpath of the JVM. Application-specific classloaders (including WebLogic Server classloaders) are children of the system classpath classloader.系统classpath类装载器继承了JDK扩展类装载器。

系统类装载器从JVM指定的应用类装载器（系统装载器的子类装载器）的classpth来加载类。

Note: What BEA refers to as a "system classpath classloader" is often referred to as the "application classloader" in contexts outside of WebLogic Server. When discussing classloaders in WebLogic Server, BEA uses the term "system" to differentiate from classloaders related to J2EE applications (which BEA refers to as "application classloaders").注意：在WebLogic应用服务器之外，BEA中说的“系统classpth类装载器”也称为“应用类装载器”。

在weblogic中讨论类装载器的时候，BEA使用的术语“系统”和J2EE应用服务器的类装载器（BEA称之为“应用类装载器”）是有区别的。

Loading a ClassClassloaders use a delegation model when loading a class. The classloader implementation first checks its cache to see if the requested class has already been loaded. This class verification improves performance in that its cached memory copy is used instead of repeated loading of a class from disk. If the class is not found in its cache, the current classloader asks its parent for the class. Only if the parent cannot load the class does the classloader attempt to load the class. If a class exists in both the parent and child classloaders, the parent version is loaded. This delegation model is followed to avoid multiple copies of the same form being loaded. Multiple copies of the same class can lead to a ClassCastException.类装载器使用代理模式来装载一个类。

子类装载器时候从它的缓存中检查被请求的类是否已经装载。

类被缓存在内存中而不是重复的从硬盘上读取，这样可以提高类的校验效率。

如果被请求的类不在它的缓存中，那么当前的类装载器就请求它的父类来装载这个类。

当父类装载器无法找到请求的类，那么子类装载器才试图去装载这个类。

如果一个类既在父类装载器中又在子类装载器中，那么父类装载器中的类会被加载。

代理模式可以防止同样的类被多次装载。

多次装载相同的类会产生ClassCastException异常。

Classloaders ask their parent classloader to load a class before attempting to load the class themselves. Classloaders in WebLogic Server that are associated with Web applications can be configured to check locally first before asking their parent for the class. This allows Web applications to use their own versions of third-party classes, which might also be used as part of the WebLogic Server product. Theprefer-web-inf-classes Element section discusses this in more detail.类装载器在自己装载请求类之前，先请求他们的父类装载器来装载此类。