Loading of Classes and Interfaces
12.2 Loading of Classes and Interfaces
Loading refers to the process of finding the binary form of a class or interface type with a particular name, perhaps by computing it on the fly, but more typically by retrieving a binary representation previously computed from source code by a Java compiler, and constructing, from that binary form, a Class object to represent the class or interface.
The precise semantics of loading are given in Chapter 5 of The Java™ Virtual Machine Specification, Java SE 7 Edition. Here we present an overview of the process from the viewpoint of the Java programming language.
The binary format of a class or interface is normally the class file format described in The Java™ Virtual Machine Specification, Java SE 7 Edition cited above, but other formats are possible, provided they meet the requirements specified in §13.1. The method defineClass of class ClassLoader may be used to construct Class objects from binary representations in the class file format.
Well-behaved class loaders maintain these properties:
EXECUTION The Loading Process 12.2.1
• Given the same name, a good class loader should always return the same class object.
• If a class loader L1 delegates loading of a class C to another loader L2 , then for any type T that occurs as the direct superclass or a direct superinterface of C , or as the type of a field in C , or as the type of a formal parameter of a method or constructor in C , or as a return type of a method in C , L1 and L2 should return the same Class object.
A malicious class loader could violate these properties. However, it could not undermine the security of the type system, because the Java virtual machine guards against this.
For further discussion of these issues, see The Java™ Virtual Machine Specification, Java SE 7 Edition and the paper Dynamic Class Loading in the Java Virtual Machine, by Sheng Liang and Gilad Bracha, in Proceedings of OOPSLA '98, published as ACM SIGPLAN Notices, Volume 33, Number 10, October 1998, pages 36-44. A basic principle of the design of the Java programming language is that the run-time type system cannot be subverted by code written in the Java programming language, not even by implementations of such otherwise sensitive system classes as ClassLoader and SecurityManager .
12.2.1 The Loading Process
The loading process is implemented by the class ClassLoader and its subclasses. Different subclasses of ClassLoader may implement different loading policies. In
particular, a class loader may cache binary representations of classes and interfaces, prefetch them based on expected usage, or load a group of related classes together. These activities may not be completely transparent to a running application if, for example, a newly compiled version of a class is not found because an older version is cached by a class loader. It is the responsibility of a class loader, however, to reflect loading errors only at points in the program where they could have arisen without prefetching or group loading.
If an error occurs during class loading, then an instance of one of the following subclasses of class LinkageError will be thrown at any point in the program that (directly or indirectly) uses the type:
• ClassCircularityError : A class or interface could not be loaded because it would be its own superclass or superinterface (§8.1.4, §9.1.3, §13.4.4).
• ClassFormatError : The binary data that purports to specify a requested compiled class or interface is malformed.
• NoClassDefFoundError : No definition for a requested class or interface could
be found by the relevant class loader.
12.3 Linking of Classes and Interfaces EXECUTION
Because loading involves the allocation of new data structures, it may fail with an OutOfMemoryError .