manual.html

一个用于对.class文件进行插桩的开源工具

     <a href="#Figure 3">Figure 3</a> shows an UML diagram of the
     hierarchy of classes of the <font face="helvetica,arial">BCEL
     </font>API. <a href="#Figure 8">Figure 8</a> in the appendix also
     shows a detailed diagram of the <tt>ConstantPool</tt> components.
  </p>
                                                <p align="center">
  <a name="Figure 3">
  <img src="images/javaclass.gif" /> <br />
  Figure 3: UML diagram for the JavaClass API</a>
  </p>
                                                <p>
    The top-level data structure is <tt>JavaClass</tt>, which in most
    cases is created by a <tt>ClassParser</tt> object that is capable
    of parsing binary class files. A <tt>JavaClass</tt> object
    basically consists of fields, methods, symbolic references to the
    super class and to the implemented interfaces.
  </p>
                                                <p>
    The constant pool serves as some kind of central repository and is
    thus of outstanding importance for all components.
    <tt>ConstantPool</tt> objects contain an array of fixed size of
    <tt>Constant</tt> entries, which may be retrieved via the
    <tt>getConstant()</tt> method taking an integer index as argument.
    Indexes to the constant pool may be contained in instructions as
    well as in other components of a class file and in constant pool
    entries themselves.
  </p>
                                                <p>
    Methods and fields contain a signature, symbolically defining
    their types.  Access flags like <tt>public static final</tt> occur
    in several places and are encoded by an integer bit mask, e.g.,
    <tt>public static final</tt> matches to the Java expression
  </p>
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      <td bgcolor="#ffffff"><pre>int access_flags = ACC_PUBLIC | ACC_STATIC | ACC_FINAL;</pre></td>
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                                                <p>
    As mentioned in <a href="#2.1 Java class file format">section
    2.1</a> already, several components may contain <em>attribute</em>
    objects: classes, fields, methods, and <tt>Code</tt> objects
    (introduced in <a href="#2.3 Method code">section 2.3</a>).  The
    latter is an attribute itself that contains the actual byte code
    array, the maximum stack size, the number of local variables, a
    table of handled exceptions, and some optional debugging
    information coded as <tt>LineNumberTable</tt> and
    <tt>LocalVariableTable</tt> attributes. Attributes are in general
    specific to some data structure, i.e., no two components share the
    same kind of attribute, though this is not explicitly
    forbidden. In the figure the <tt>Attribute</tt> classes are stereotyped
    with the component they belong to.
  </p>
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        <font color="#ffffff" face="arial,helvetica,sanserif">
          <a name="3.2 Class repository"><strong>3.2 Class repository</strong></a>
        </font>
      </td></tr>
      <tr><td>
        <blockquote>
                                    <p>
    Using the provided <tt>Repository</tt> class, reading class files into
    a <tt>JavaClass</tt> object is quite simple:
  </p>
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      <td bgcolor="#ffffff"><pre>JavaClass clazz = Repository.lookupClass(&quot;java.lang.String&quot;);</pre></td>
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                                                <p>
    The repository also contains methods providing the dynamic equivalent
    of the <tt>instanceof</tt> operator, and other useful routines:
  </p>
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  if(Repository.instanceOf(clazz, super_class) {
    ...
  }</pre></td>
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                            </blockquote>
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        <font color="#ffffff" face="arial,helvetica,sanserif">
          <a name="3.2.1 Accessing class file data"><strong>3.2.1 Accessing class file data</strong></a>
        </font>
      </td></tr>
      <tr><td>
        <blockquote>
                                    <p>
    Information within the class file components may be accessed like
    Java Beans via intuitive set/get methods. All of them also define
    a <tt>toString()</tt> method so that implementing a simple class
    viewer is very easy. In fact all of the examples used here have
    been produced this way:
  </p>
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      <td bgcolor="#ffffff"><pre>
  System.out.println(clazz);
  printCode(clazz.getMethods());
  ...
  public static void printCode(Method[] methods) {
    for(int i=0; i &lt; methods.length; i++) {
      System.out.println(methods[i]);

      Code code = methods[i].getCode();
      if(code != null) // Non-abstract method
        System.out.println(code);
    }
  }
  </pre></td>
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                            </blockquote>
        </p>
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        <font color="#ffffff" face="arial,helvetica,sanserif">
          <a name="3.2.2 Analyzing class data"><strong>3.2.2 Analyzing class data</strong></a>
        </font>
      </td></tr>
      <tr><td>
        <blockquote>
                                    <p>
    Last but not least, <font face="helvetica,arial">BCEL</font>
    supports the <em>Visitor</em> design pattern, so one can write
    visitor objects to traverse and analyze the contents of a class
    file. Included in the distribution is a class
    <tt>JasminVisitor</tt> that converts class files into the <a href="http://mrl.nyu.edu/~meyer/jasmin/">Jasmin</a>
    assembler language.
  </p>
                            </blockquote>
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        <font color="#ffffff" face="arial,helvetica,sanserif">
          <a name="3.3 ClassGen"><strong>3.3 ClassGen</strong></a>
        </font>
      </td></tr>
      <tr><td>
        <blockquote>
                                    <p>
    This part of the API (package <tt>org.apache.bcel.generic</tt>)
    supplies an abstraction level for creating or transforming class
    files dynamically. It makes the static constraints of Java class
    files like the hard-coded byte code addresses "generic". The
    generic constant pool, for example, is implemented by the class
    <tt>ConstantPoolGen</tt> which offers methods for adding different
    types of constants. Accordingly, <tt>ClassGen</tt> offers an
    interface to add methods, fields, and attributes.
     <a href="#Figure 4">Figure 4</a> gives an overview of this part of the API.
  </p>
                                                <p align="center">
    <a name="Figure 4">
    <img src="images/classgen.gif" />
    <br />
    Figure 4: UML diagram of the ClassGen API</a>
  </p>
                            </blockquote>
        </p>
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        <font color="#ffffff" face="arial,helvetica,sanserif">
          <a name="3.3.1 Types"><strong>3.3.1 Types</strong></a>
        </font>
      </td></tr>
      <tr><td>
        <blockquote>
                                    <p>
    We abstract from the concrete details of the type signature syntax
    (see <a href="#2.5 Type information">2.5</a>) by introducing the
    <tt>Type</tt> class, which is used, for example, by methods to
    define their return and argument types. Concrete sub-classes are
    <tt>BasicType</tt>, <tt>ObjectType</tt>, and <tt>ArrayType</tt>
    which consists of the element type and the number of
    dimensions. For commonly used types the class offers some
    predefined constants. For example, the method signature of the
    <tt>main</tt> method as shown in 
    <a href="#2.5 Type information">section 2.5</a> is represented by:
  </p>