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.
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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.
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 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.