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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><!-- Content Stylesheet for Site --> <!-- start the processing --> <!-- ====================================================================== --> <!-- Main Page Section --> <!-- ====================================================================== --> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"/> <meta name="author" value="Markus Dahm"> <meta name="email" value="markus.dahm@berlin.de"> <title>BCEL - Byte Code Engineering Library (BCEL)</title> </head> <body bgcolor="#ffffff" text="#000000" link="#525D76"> <table border="0" width="100%" cellspacing="0"> <!-- TOP IMAGE --> <tr> <td align="left"><a href="http://jakarta.apache.org"><img src="http://jakarta.apache.org/images/jakarta-logo.gif" border="0"/></a></td><td align="right"><a href="http://jakarta.apache.org/bcel"><img src="./images/bcel-logo.gif" alt="BCEL" border="0"/></a></td> </tr> </table> <table border="0" width="100%" cellspacing="4"> <tr><td colspan="2"> <hr noshade="" size="1"/> </td></tr> <tr> <!-- LEFT SIDE NAVIGATION --> <td width="20%" valign="top" nowrap="true"> <p><strong>BCEL</strong></p> <ul> <li> <a href="./index.html">Front Page</a></li> <li> <a href="./news.html">News & Status</a></li> </ul> <p><strong>Documentation</strong></p> <ul> <li> <a href="./manual.html">Manual</a></li> <li> <a href="./faq.html">FAQ</a></li> </ul> <p><strong>Download</strong></p> <ul> <li> <a href="http://jakarta.apache.org/site/binindex.html">Binaries</a></li> <li> <a href="http://jakarta.apache.org/site/sourceindex.html">Source Code</a></li> </ul> <p><strong>Get Involved</strong></p> <ul> <li> <a href="http://cvs.apache.org/viewcvs/jakarta-bcel/">CVS Repository</a></li> <li> <a href="http://jakarta.apache.org/site/mail.html">Mailing Lists</a></li> <li> <a href="http://jakarta.apache.org/site/bugs.html">Issue Tracking</a></li> <li> <a href="./contributors.html">Contributors</a></li> <li> <a href="./license.html">License</a></li> <li> <a href="./projects.html">Projects</a></li> </ul> </td> <td width="80%" align="left" valign="top"> <table border="0" cellspacing="0" cellpadding="2" width="100%"> <tr><td bgcolor="#525D76"> <font color="#ffffff" face="arial,helvetica,sanserif"> <a name="Abstract"><strong>Abstract</strong></a> </font> </td></tr> <tr><td> <blockquote> <p> Extensions and improvements of the programming language Java and its related execution environment (Java Virtual Machine, JVM) are the subject of a large number of research projects and proposals. There are projects, for instance, to add parameterized types to Java, to implement <a href="http://aspectj.org/">Aspect-Oriented Programming</a>, to perform sophisticated static analysis, and to improve the run-time performance. </p> <p> Since Java classes are compiled into portable binary class files (called <em>byte code</em>), it is the most convenient and platform-independent way to implement these improvements not by writing a new compiler or changing the JVM, but by transforming the byte code. These transformations can either be performed after compile-time, or at load-time. Many programmers are doing this by implementing their own specialized byte code manipulation tools, which are, however, restricted in the range of their re-usability. </p> <p> To deal with the necessary class file transformations, we introduce an API that helps developers to conveniently implement their transformations. </p> </blockquote> </p> </td></tr> <tr><td><br/></td></tr> </table> <table border="0" cellspacing="0" cellpadding="2" width="100%"> <tr><td bgcolor="#525D76"> <font color="#ffffff" face="arial,helvetica,sanserif"> <a name="1 Introduction"><strong>1 Introduction</strong></a> </font> </td></tr> <tr><td> <blockquote> <p> The <a href="http://java.sun.com/">Java</a> language has become very popular and many research projects deal with further improvements of the language or its run-time behavior. The possibility to extend a language with new concepts is surely a desirable feature, but the implementation issues should be hidden from the user. Fortunately, the concepts of the Java Virtual Machine permit the user-transparent implementation of such extensions with relatively little effort. </p> <p> Because the target language of Java is an interpreted language with a small and easy-to-understand set of instructions (the <em>byte code</em>), developers can implement and test their concepts in a very elegant way. One can write a plug-in replacement for the system's <em>class loader</em> which is responsible for dynamically loading class files at run-time and passing the byte code to the Virtual Machine (see section ). Class loaders may thus be used to intercept the loading process and transform classes before they get actually executed by the JVM. While the original class files always remain unaltered, the behavior of the class loader may be reconfigured for every execution or instrumented dynamically. </p> <p> The <font face="helvetica,arial">BCEL</font> API (Byte Code Engineering Library), formerly known as JavaClass, is a toolkit for the static analysis and dynamic creation or transformation of Java class files. It enables developers to implement the desired features on a high level of abstraction without handling all the internal details of the Java class file format and thus re-inventing the wheel every time. <font face="helvetica,arial">BCEL </font> is written entirely in Java and freely available under the terms of the <a href="license.html">Apache Software License</a>. </p> <p> This manual is structured as follows: We give a brief description of the Java Virtual Machine and the class file format in <a href="#2 The Java Virtual Machine">section 2</a>. <a href="#3 The BCEL API">Section 3</a> introduces the <font face="helvetica,arial">BCEL</font> API. <a href="#4 Application areas">Section 4</a> describes some typical application areas and example projects. The appendix contains code examples that are to long to be presented in the main part of this paper. All examples are included in the down-loadable distribution. </p> </blockquote> </p> </td></tr> <tr><td><br/></td></tr> </table> <table border="0" cellspacing="0" cellpadding="2" width="100%"> <tr><td bgcolor="#525D76"> <font color="#ffffff" face="arial,helvetica,sanserif"> <a name="2 The Java Virtual Machine"><strong>2 The Java Virtual Machine</strong></a> </font> </td></tr> <tr><td> <blockquote> <p> Readers already familiar with the Java Virtual Machine and the Java class file format may want to skip this section and proceed with <a href="#3 The BCEL API">section 3</a>. </p> <p> Programs written in the Java language are compiled into a portable binary format called <em>byte code</em>. Every class is represented by a single class file containing class related data and byte code instructions. These files are loaded dynamically into an interpreter (<a href="http://java.sun.com/docs/books/vmspec/index.html">Java Virtual Machine</a>, aka. JVM) and executed. </p> <p> <a href="#Figure 1">Figure 1</a> illustrates the procedure of compiling and executing a Java class: The source file (<tt>HelloWorld.java</tt>) is compiled into a Java class file (<tt>HelloWorld.class</tt>), loaded by the byte code interpreter and executed. In order to implement additional features, researchers may want to transform class files (drawn with bold lines) before they get actually executed. This application area is one of the main issues of this article. </p> <p align="center"> <a name="Figure 1"> <img src="images/jvm.gif" /> <br /> Figure 1: Compilation and execution of Java classes</a> </p> <p> Note that the use of the general term "Java" implies in fact two meanings: on the one hand, Java as a programming language, on the other hand, the Java Virtual Machine, which is not necessarily targeted by the Java language exclusively, but may be used by <a href="http://grunge.cs.tu-berlin.de/~tolk/vmlanguages.html">other languages</a> as well. We assume the reader to be familiar with the Java language and to have a general understanding of the Virtual Machine. </p> </blockquote> </p> </td></tr> <tr><td><br/></td></tr> </table> <table border="0" cellspacing="0" cellpadding="2" width="100%"> <tr><td bgcolor="#525D76"> <font color="#ffffff" face="arial,helvetica,sanserif"> <a name="2.1 Java class file format"><strong>2.1 Java class file format</strong></a> </font> </td></tr> <tr><td> <blockquote> <p> Giving a full overview of the design issues of the Java class file format and the associated byte code instructions is beyond the scope of this paper. We will just give a brief introduction covering the details that are necessary for understanding the rest of this paper. The format of class files and the byte code instruction set are described in more detail in the <a href="http://java.sun.com/docs/books/vmspec/index.html">Java Virtual Machine Specification</a>. Especially, we will not deal with the security constraints that the Java Virtual Machine has to check at run-time, i.e. the byte code verifier. </p> <p> <a href="#Figure 2">Figure 2</a> shows a simplified example of the contents of a Java class file: It starts with a header containing a "magic number" (<tt>0xCAFEBABE</tt>) and the version number, followed by the <em>constant pool</em>, which can be roughly thought of as the text segment of an executable, the <em>access rights</em> of the class encoded by a bit mask, a list of interfaces implemented by the class, lists containing the fields and methods of the class, and finally the <em>class attributes</em>, e.g., the <tt>SourceFile</tt> attribute telling the name of the source file. Attributes are a way of putting additional, user-defined information into class file data structures. For example, a custom class loader may evaluate such attribute data in order to perform its transformations. The JVM specification declares that unknown, i.e., user-defined attributes must be ignored by any Virtual Machine implementation. </p> <p align="center">⌨️ 快捷键说明
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