manual.html

一个用于对.class文件进行插桩的开源工具

    integer number. Within signatures they are represented by single
    characters, e.g., <tt>I</tt>, for integer. Arrays are denoted with
    a <tt>[</tt> at the start of the signature.
  </p>
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          <a name="2.6 Code example"><strong>2.6 Code example</strong></a>
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        <blockquote>
                                    <p>
    The following example program prompts for a number and prints the
    faculty of it. The <tt>readLine()</tt> method reading from the
    standard input may raise an <tt>IOException</tt> and if a
    misspelled number is passed to <tt>parseInt()</tt> it throws a
    <tt>NumberFormatException</tt>. Thus, the critical area of code
    must be encapsulated in a <tt>try-catch</tt> block.
  </p>
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      <td bgcolor="#ffffff"><pre>  
    import java.io.*;

    public class Faculty {
      private static BufferedReader in = new BufferedReader(new
                                InputStreamReader(System.in));

      public static final int fac(int n) {
        return (n == 0)? 1 : n * fac(n - 1);
      }

      public static final int readInt() {
        int n = 4711;
        try {
        System.out.print(&quot;Please enter a number&gt; &quot;);
        n = Integer.parseInt(in.readLine());
        } catch(IOException e1) { System.err.println(e1); }
        catch(NumberFormatException e2) { System.err.println(e2); }
        return n;
      }

      public static void main(String[] argv) {
        int n = readInt();
        System.out.println(&quot;Faculty of &quot; + n + &quot; is &quot; + fac(n));
      }
    }
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                                                <p>
    This code example typically compiles to the following chunks of
    byte code:
  </p>
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    0:  iload_0
    1:  ifne            #8
    4:  iconst_1
    5:  goto            #16
    8:  iload_0
    9:  iload_0
    10: iconst_1
    11: isub
    12: invokestatic    Faculty.fac (I)I (12)
    15: imul
    16: ireturn

    LocalVariable(start_pc = 0, length = 16, index = 0:int n)
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                                                <p><b>fac():</b>
    The method <tt>fac</tt> has only one local variable, the argument
    <tt>n</tt>, stored at index 0. This variable's scope ranges from
    the start of the byte code sequence to the very end.  If the value
    of <tt>n</tt> (the value fetched with <tt>iload_0</tt>) is not
    equal to 0, the <tt>ifne</tt> instruction branches to the byte
    code at offset 8, otherwise a 1 is pushed onto the operand stack
    and the control flow branches to the final return.  For ease of
    reading, the offsets of the branch instructions, which are
    actually relative, are displayed as absolute addresses in these
    examples.
  </p>
                                                <p>
    If recursion has to continue, the arguments for the multiplication
    (<tt>n</tt> and <tt>fac(n - 1)</tt>) are evaluated and the results
    pushed onto the operand stack.  After the multiplication operation
    has been performed the function returns the computed value from
    the top of the stack.
  </p>
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      <td bgcolor="#ffffff"><pre>
    0:  sipush        4711
    3:  istore_0
    4:  getstatic     java.lang.System.out Ljava/io/PrintStream;
    7:  ldc           &quot;Please enter a number&gt; &quot;
    9:  invokevirtual java.io.PrintStream.print (Ljava/lang/String;)V
    12: getstatic     Faculty.in Ljava/io/BufferedReader;
    15: invokevirtual java.io.BufferedReader.readLine ()Ljava/lang/String;
    18: invokestatic  java.lang.Integer.parseInt (Ljava/lang/String;)I
    21: istore_0
    22: goto          #44
    25: astore_1
    26: getstatic     java.lang.System.err Ljava/io/PrintStream;
    29: aload_1
    30: invokevirtual java.io.PrintStream.println (Ljava/lang/Object;)V
    33: goto          #44
    36: astore_1
    37: getstatic     java.lang.System.err Ljava/io/PrintStream;
    40: aload_1
    41: invokevirtual java.io.PrintStream.println (Ljava/lang/Object;)V 
    44: iload_0
    45: ireturn

    Exception handler(s) = 
    From    To      Handler Type
    4       22      25      java.io.IOException(6)
    4       22      36      NumberFormatException(10)
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                                                <p><b>readInt():</b> First the local variable <tt>n</tt> (at index 0)
    is initialized to the value 4711.  The next instruction,
    <tt>getstatic</tt>, loads the referencs held by the static
    <tt>System.out</tt> field onto the stack. Then a string is loaded
    and printed, a number read from the standard input and assigned to
    <tt>n</tt>.
  </p>
                                                <p>
    If one of the called methods (<tt>readLine()</tt> and
    <tt>parseInt()</tt>) throws an exception, the Java Virtual Machine
    calls one of the declared exception handlers, depending on the
    type of the exception.  The <tt>try</tt>-clause itself does not
    produce any code, it merely defines the range in which the
    subsequent handlers are active. In the example, the specified
    source code area maps to a byte code area ranging from offset 4
    (inclusive) to 22 (exclusive).  If no exception has occurred
    ("normal" execution flow) the <tt>goto</tt> instructions branch
    behind the handler code. There the value of <tt>n</tt> is loaded
    and returned.
  </p>
                                                <p>
    The handler for <tt>java.io.IOException</tt> starts at
    offset 25. It simply prints the error and branches back to the
    normal execution flow, i.e., as if no exception had occurred.
  </p>
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          <a name="3 The BCEL API"><strong>3 The BCEL API</strong></a>
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        <blockquote>
                                    <p>
    The <font face="helvetica,arial">BCEL</font> API abstracts from
    the concrete circumstances of the Java Virtual Machine and how to
    read and write binary Java class files. The API mainly consists
    of three parts:
  </p>
                                                <p>

    <ol type="1">
    <li> A package that contains classes that describe "static"
    constraints of class files, i.e., reflects the class file format and
    is not intended for byte code modifications. The classes may be
    used to read and write class files from or to a file.  This is
    useful especially for analyzing Java classes without having the
    source files at hand.  The main data structure is called
    <tt>JavaClass</tt> which contains methods, fields, etc..</li>

    <li> A package to dynamically generate or modify
    <tt>JavaClass</tt> or <tt>Method</tt> objects.  It may be used to
    insert analysis code, to strip unnecessary information from class
    files, or to implement the code generator back-end of a Java
    compiler.</li>

    <li> Various code examples and utilities like a class file viewer,
    a tool to convert class files into HTML, and a converter from
    class files to the <a href="http://mrl.nyu.edu/~meyer/jasmin/">Jasmin</a> assembly
    language.</li>
    </ol>
  </p>
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          <a name="3.1 JavaClass"><strong>3.1 JavaClass</strong></a>
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        <blockquote>
                                    <p>
    The "static" component of the <font face="helvetica,arial">BCEL</font> API resides in the package
     <tt>org.apache.bcel.classfile</tt> and closely represents class
     files. All of the binary components and data structures declared
     in the <a href="http://java.sun.com/docs/books/vmspec/index.html">JVM
     specification</a> and described in section <a href="#2 The Java Virtual Machine">2</a> are mapped to classes.