diff --git a/xdocs/index.xml b/xdocs/index.xml index 315e2ea..bedd958 100644 --- a/xdocs/index.xml +++ b/xdocs/index.xml @@ -54,6 +54,11 @@ In particular, you should read the package overview of the org.apache.comm package. In addition, there is a (short) User Guide.

+

+Users needing to become experts or wanting to help develop JCL should +(in addition) consult the Tech Guide. +This gives short introductions to topics such as advanced class loading. +

diff --git a/xdocs/navigation.xml b/xdocs/navigation.xml index aa508fa..bbee620 100755 --- a/xdocs/navigation.xml +++ b/xdocs/navigation.xml @@ -36,6 +36,8 @@ href="/index.html"/> + diff --git a/xdocs/tech.xml b/xdocs/tech.xml new file mode 100644 index 0000000..18b5c37 --- /dev/null +++ b/xdocs/tech.xml @@ -0,0 +1,579 @@ + + + + + + + + JCL Technology Guide + Commons Documentation Team + + + +
+ + + + +

+ This guide is aimed at describing the technologies that JCL developers and expert users + (and users who need to become experts) + should be familiar with. The aim is to give an understanding whilst being precise but brief. + Details which are not relevent for JCL have been suppressed. + References have been included. +

+

+ These topics are a little difficult and it's easy for even experience developers to make + mistakea. We need you to help us get it right! Please submit corrections, comments, additional references + and requests for clarification + by either: +

+ +

+ TIA +

+ + +
+
+ +

+ This is intended to present a guide to the process by which Java bytecode uses bytecode in other classes + from the perspective of the language and virtual machine specifications. The focus will be on deciding + which bytecode will be used (rather than the mechanics of the usage). It focusses on facts and terminology. +

+

+ The process is recursive: it is therefore difficult to pick a starting point. + Sun's documentation starts from the persective of the startup of a new application. + This guide starts from the perspective of an executing application. +

+

+ During this discussion, please assume that each time that class is mentioned, + the comments applied equally well to interfaces. +

+

+ This document is targeted at Java 1.2 and above. +

+ + +

+ (LangSpec 12.3.3) + The bytecode representation of a class contains symbolic names for other classes referenced. +

+

+ + In practical development terms: If a class is imported (either explicitly in the list of imports at the top of + the source file or implicitly through a fully qualified name in the source code) it is referenced symbolically. + +

+

+ (VMSpec 5.4.3) + Resolution of a symbolic reference occurs dymanically at runtime and is carried out by + the Java Vitual Machine. Resolution of a symbolic reference requires loading and linking of the new class. +

+

+ + Note: references are not statically resolved at compile time. + +

+ + +

+ (VMSpec 2.17.2) + Loading is the name given process by which a binary form of a class is obtained + by the Java Virutal Machine. + Java classes are always loaded and linked dynamically by the Java Virtual Machine + (rather than statically by the compiler). +

+

+ + In practical development terms: + This means that the developer has no certain knowledge about the actual + bytecode that will be used to execute any external call (one made outside the class). This is determined only + at execution time and is effected by the way that the code is deployed. + +

+ + +

+ (VMSpec 2.17.3) + Linking is the name used for combining the + binary form of a class into the Java Virtual Machine. This must happen before the class can be used. +

+

+ (VMSpec 2.17.3) + Linking is composed of verification, preparation and resolution (of symbolic references). + Flexibility is allowed over the timing of resolution. (Within limit) this may happen at any time after + preparation and before that reference is used. +

+

+ + In practical development terms: This means that different JVMs may realize that a reference cannot be + resolved at different times during execution. Consequently, the actual behaviour cannot be precisely predicted + without intimate knowledge of the JVM (on which the bytecode will be executed). + This makes it hard to give universal guildance to users. + +

+ + + +

+ (VMSpec 2.17.2) + The loading process is performed by a ClassLoader. +

+

+ (VMSpec 5.3) + A classloader may create a class either by delegation or by defining it directly. + The classloader that initiates loading of a class is known as the initiating loader. + The classloader that defines the class is known as the defining loader. +

+

+ + In practical terms: understanding and appreciating this distinction is crucial when debugging issues + concerning classloaders. + +

+ + + +

+ (VMSPEC 5.3) + The bootstrap is the base ClassLoader supplied by the Java Virtual Machine. + All others are user (also known as application) ClassLoader's. +

+

+ + In practical development terms: The System classloader returned by Classloader.getSystemClassLoader() + will be either the bootstrap classloader or a direct descendent of the bootstrap classloader. + Only when debugging issues concerning the system classloader should there be any need to consider the detailed + differences between the bootstrap classloader and the system classloader. + +

+ + +

+ (VMSpec 5.3) + At runtime, a class (or interface) is determined by it's fully qualified name + and by the classloader that defines it. This is known as the class's runtime package. +

+

+ (VMSpec 5.4.4) + Only classes in the same runtime package are mutually accessible. +

+

+ + In practical development terms: two classes with the same symbolic name can only be used interchangably + if they are defined by the same classloader. A classic symptom indicative of a classloader issue is that + two classes with the same fully qualified name are found to be incompatible during a method call. + This may happen when a member is expecting an interface which is (seemingly) implemented by a class + but the class is in a different runtime package after being defined by a different classloader. This is a + fundemental java language security feature. + +

+ + + +

+ (VMSpec 5.3) + The classloader which defines the class (whose reference is being resolved) is the one + used to initiate loading of the class referred to. +

+

+ + In practial development terms: This is very important to bear in mind when trying to solve classloader issues. + A classic misunderstanding is this: suppose class A defined by classloader C has a symbolic reference to + class B and further that when C initiates loading of B, this is delegated to classloader D which defines B. + Class B can now only resolve symbols that can be loaded by D, rather than all those which can be loaded by C. + This is a classic recipe for classloader problems. + +

+ + + +
    +
  • + VMSpec The Java Virtual Machine Specification, Second Edition +
    • +
  • + LangSpec The Java Language Specification Second Edition +
    • +
+ +
+
+ +

+ When asked to load a class, a class loader may either define the class itself or delegate. + The base ClassLoader class insists that every implementation has a parent class loader. + This delegation model therefore naturally forms a tree structure rooted in the bootstrap classloader. +

+

+ Containers often use complex trees to allow isolation of different applications + running within the container. + This is particularly true of J2EE containers +

+ + + +

+ When a classloader is asked to load a class, a question presents itself: should it immediately + delegate the loading to it's parent (and thus only define those classes not defined by it's parent) + or should it try to define it first itself (and only delegate to it's parent those classes it does + not itself define). Classloaders which universally adopt the first approach are termed parent-first + and the second child-first. +

+

+ Though child-first and parent-first are not the only strategies possible, + they are by far the most common. + All other strategies are rare. + However, it is not uncommon to be faced with a mixture of parent-first and child-first + classloaders within the same hierarchy. +

+ + + +

+ The class loader used to define the class is available programmatically by calling + the getClassLoader method + on the class in question. This is often known as the class classloader. +

+ + + +

+ Java 1.2 introduces a mechanism which allows code to access classloaders + which are not parents of the class classloader. + A thread may have a class loader associated to it by it's creator for use + by code running in this thread when loading resources and classes. + This is accessed by the getContextClassLoader method on Thread. + This is therefore often known as the context classloader. +

+

+ Note that the quality and appropriateness of the context classloader depends on the + care with which the thread is created. +

+ + + +

+ In practice, context classloaders vary in quality and issues sometimes arise + when using them. + The creator of the thread is responsible for setting the classloader. + If the context clasloader is not set then it will default to the system + classloader. + Any container doing so will cause difficulties for any code using the context classloader. +

+

+ The creator is also at liberty to set the classloader as they wish. + Containers may set the context classloader so that it is nether a child nor a parent + of the classloader that defines the class using that loader. + Again, this will cause difficulties. +

+ + + +

+ Introduced in Java J2EE 1.3 + is a requirement for vendors to appropriately set the context classloader. + Section 6.2.4.8 (1.4 text): +

+ +This specification requires that J2EE containers provide a per thread +context class loader for the use of system or library classes in +dynamicly loading classes provided by the application. The EJB +specification requires that all EJB client containers provide a per +thread context class loader for dynamicly loading system value classes. +The per thread context class loader is accessed using the Thread method +getContextClassLoader. + +The classes used by an application will typically be loaded by a +hierarchy of class loaders. There may be a top level application class +loader, an extension class loader, and so on, down to a system class +loader. The top level application class loader delegates to the lower +class loaders as needed. Classes loaded by lower class loaders, such as +portable EJB system value classes, need to be able to discover the top +level application class loader used to dynamicly load application +classes. + +We require that containers provide a per thread context class loader +that can be used to load top level application classes as described +above. + +

+ This specification leaves quite a lot of freedom for vendors. + (As well as using unconventional terminology and containing the odd typo.) + It is a difficult passage (to say the least). +

+ + + +

+ Reflection cannot bypass restrictions imposed by the java langauge security model but by avoiding symbolic + references, reflection can be used to load classes which could not otherwise be loaded. Another ClassLoader + can be used to load a class and then reflection used to create an instance. +

+

+ Recall that the runtime packaging is used to determine accessibility. + Reflection cannot be used to avoid basic java security. + Therefore, the runtime packaging becomes an issue when attempting to cast classes + created by reflection using other class loaders. + When using this strategy, various modes of failure are possible + when common class references are defined by the different class loaders. +

+

+ Reflection is often used with the context classloader. In theory, this allows a class defined in + a parent classloader to load any class that is loadable by the application. + In practice, this only works well when the context classloader is set carefully. +

+ + + + + +
+ +
+ +

+ JCL takes the view that different context class loader indicate boundaries between applications + running in a container environment. Isolation requires that JCL honours these boundaries + and therefore allows different isolated applications to configure their logging systems + independently. +

+ + +

+ Performance dictates that symbolic references to these classes are present in the calling application code + (reflection would simply be too slow). Therefore, these classes must be loadable by the classloader + that loads the application code. +

+ + +

+ Performance dictates that symbolic references to the logging systems are present in the implementation + classes (again, reflection would simply be too slow). So, for an implementation to be able to function, + it is neccessary for the logging system to be loadable by the classloader that defines the implementing class. +

+ + + +

+ However, there is actually no reason why LogFactory requires symbolic references to particular Log + implementations. Reflection can be used to load these from an appropriate classloader + without unacceptable performance degradation. + This is the strategy adopted by JCL. +

+

+ JCL uses the context classloader to use the Log implementation. +

+ +
+ + \ No newline at end of file