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See the License for the 17 * specific language governing permissions and limitations 18 * under the License. 19 --> 20 <document> 21 <properties> 22 <title>Application areas</title> 23 </properties> 24 25 <body> 26 <section name="Application areas"> 27 <p> 28 There are many possible application areas for <font 29 face="helvetica,arial">BCEL</font> ranging from class 30 browsers, profilers, byte code optimizers, and compilers to 31 sophisticated run-time analysis tools and extensions to the Java 32 language. 33 </p> 34 35 <p> 36 Compilers like the <a 37 href="http://barat.sourceforge.net">Barat</a> compiler use <font 38 face="helvetica,arial">BCEL</font> to implement a byte code 39 generating back end. Other possible application areas are the 40 static analysis of byte code or examining the run-time behavior of 41 classes by inserting calls to profiling methods into the 42 code. Further examples are extending Java with Eiffel-like 43 assertions, automated delegation, or with the concepts of <a 44 href="http://www.eclipse.org/aspectj/">Aspect-Oriented Programming</a>.<br/> A 45 list of projects using <font face="helvetica,arial">BCEL</font> can 46 be found <a href="../projects.html">here</a>. 47 </p> 48 49 <subsection name="Class loaders"> 50 <p> 51 Class loaders are responsible for loading class files from the 52 file system or other resources and passing the byte code to the 53 Virtual Machine. A custom <tt>ClassLoader</tt> object may be used 54 to intercept the standard procedure of loading a class, i.e.m the 55 system class loader, and perform some transformations before 56 actually passing the byte code to the JVM. 57 </p> 58 59 <p> 60 A possible scenario is described in <a href="#Figure 7">figure 61 7</a>: 62 During run-time the Virtual Machine requests a custom class loader 63 to load a given class. But before the JVM actually sees the byte 64 code, the class loader makes a "side-step" and performs some 65 transformation to the class. To make sure that the modified byte 66 code is still valid and does not violate any of the JVM's rules it 67 is checked by the verifier before the JVM finally executes it. 68 </p> 69 70 <p align="center"> 71 <a name="Figure 7"> 72 <img src="../images/classloader.gif"/> 73 <br/> 74 Figure 7: Class loaders 75 </a> 76 </p> 77 78 <p> 79 Using class loaders is an elegant way of extending the Java 80 Virtual Machine with new features without actually modifying it. 81 This concept enables developers to use <em>load-time 82 reflection</em> to implement their ideas as opposed to the static 83 reflection supported by the <a 84 href="http://java.sun.com/j2se/1.3/docs/guide/reflection/index.html">Java 85 Reflection API</a>. Load-time transformations supply the user with 86 a new level of abstraction. He is not strictly tied to the static 87 constraints of the original authors of the classes but may 88 customize the applications with third-party code in order to 89 benefit from new features. Such transformations may be executed on 90 demand and neither interfere with other users, nor alter the 91 original byte code. In fact, class loaders may even create classes 92 <em>ad hoc</em> without loading a file at all.<br/> <font 93 face="helvetica,arial">BCEL</font> has already builtin support for 94 dynamically creating classes, an example is the ProxyCreator class. 95 </p> 96 97 </subsection> 98 99 <subsection name="Example: Poor Man's Genericity"> 100 <p> 101 The former "Poor Man's Genericity" project that extended Java with 102 parameterized classes, for example, used <font 103 face="helvetica,arial">BCEL</font> in two places to generate 104 instances of parameterized classes: During compile-time (with the 105 standard <tt>javac</tt> with some slightly changed classes) and at 106 run-time using a custom class loader. The compiler puts some 107 additional type information into class files (attributes) which is 108 evaluated at load-time by the class loader. The class loader 109 performs some transformations on the loaded class and passes them 110 to the VM. The following algorithm illustrates how the load method 111 of the class loader fulfills the request for a parameterized 112 class, e.g., <tt>Stack<String></tt> 113 </p> 114 115 <p> 116 <ol type="1"> 117 <li> Search for class <tt>Stack</tt>, load it, and check for a 118 certain class attribute containing additional type 119 information. I.e. the attribute defines the "real" name of the 120 class, i.e., <tt>Stack<A></tt>.</li> 121 122 <li>Replace all occurrences and references to the formal type 123 <tt>A</tt> with references to the actual type <tt>String</tt>. For 124 example the method 125 </li> 126 127 <source> 128 void push(A obj) { ... } 129 </source> 130 131 <p> 132 becomes 133 </p> 134 135 <source> 136 void push(String obj) { ... } 137 </source> 138 139 <li> Return the resulting class to the Virtual Machine.</li> 140 </ol> 141 </p> 142 143 </subsection> 144 </section> 145 </body> 146 </document>
