classfile.doc.html

A Java virtual machine instruction consists of an opcode specifying the operation to be performed, f

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;	<i>BaseType</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;	<i>ObjectType</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;	<i>ArrayType</i>	<br>
<a name="81068"></a>
<p>
states that a <i>FieldType</i> may represent either a <i>BaseType</i>, an <i>ObjectType</i>, or an <i>ArrayType</i>.
<p><a name="81084"></a>
A nonterminal symbol on the right-hand side of a production that is followed by an asterisk (*) represents zero or more possibly different values produced from that nonterminal, appended without any intervening space. The production:<p>
<a name="81087"></a>
<i>MethodDescriptor:</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;	( <i>ParameterDescriptor* &#32; &#32;</i>) <i>ReturnDescriptor</i><p>
<a name="81085"></a>
states that a <i>MethodDescriptor</i> represents a left parenthesis, followed by zero or 
more <i>ParameterDescriptor</i> values, followed by a right parenthesis, followed by a 
<i>ReturnDescriptor</i>.
<p><a name="14152"></a>
<h3>4.3.2    Field Descriptors</h3>
<a name="7028"></a>
A <i>field descriptor</i> represents the type of a class, instance, or local variable. It is a 
series of characters generated by the grammar:
<p><a name="13975"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>FieldDescriptor:</i><br><blockquote><a name="23539"></a>
<i>FieldType</i><p>
</blockquote><a name="23540"></a>
<p>
<a name="23546"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>ComponentType:</i><br><blockquote><a name="13979"></a>
<i>FieldType</i><p>
</blockquote><a name="23520"></a>
<p>
<a name="23545"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>FieldType:</i><br><blockquote><a name="13982"></a>
<i>BaseType</i><p>
<a name="14135"></a>
<i>ObjectType</i><p>
<a name="13983"></a>
<i>ArrayType</i>        <p>
</blockquote><a name="13984"></a>
<p>
<a name="23547"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>BaseType:</i><br><blockquote><a name="13985"></a>
B<p>
<a name="13986"></a>
C<p>
<a name="13987"></a>
D<p>
<a name="13988"></a>
F<p>
<a name="13989"></a>
I<p>
<a name="13990"></a>
J<p>
<a name="13991"></a>
S<p>
<a name="13992"></a>
Z<p>
<a name="14125"></a>
<p>
</blockquote><a name="23548"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>ObjectType:</i><br><blockquote><a name="14126"></a>
L &lt;classname&gt; ;<p>
</blockquote><a name="14127"></a>
&nbsp; &nbsp; &nbsp; &nbsp;<i>ArrayType:</i><br><blockquote><a name="14128"></a>
[ <i>ComponentType</i><p>
</blockquote><a name="14019"></a>

<a name="14019"></a>
The characters of <i>BaseType</i>, the L and ; of <i>ObjectType</i>, and the [ of <i>ArrayType</i> are all ASCII characters. The &lt;classname&gt; represents a fully qualified class or interface name. For historical reasons it is encoded in internal form (<a href="ClassFile.doc.html#14757">&#167;4.2)</a>.<p>
<a name="16252"></a>
The interpretation of the field types is as shown in <a href="ClassFile.doc.html#84645">Table 4.2</a>.<p><Table Border="1">
<tr><td><a name="84645"></a>
<i>BaseType</i><strong> Character</strong>
<td><a name="84647"></a>
<strong>Type</strong>
<td><a name="84649"></a>
<strong>Interpretation</strong>

<tr><td><a name="84652"></a>
B
<td><a name="84654"></a>
<code>byte</code>
<td><a name="84656"></a>
signed byte

<tr><td><a name="84659"></a>
C
<td><a name="84661"></a>
<code>char</code>
<td><a name="84663"></a>
Unicode character

<tr><td><a name="84666"></a>
D
<td><a name="84668"></a>
<code>double</code>
<td><a name="84670"></a>
double-precision floating-point value

<tr><td><a name="84673"></a>
F
<td><a name="84675"></a>
<code>float</code>
<td><a name="84677"></a>
single-precision floating-point value

<tr><td><a name="84681"></a>
I
<td><a name="84683"></a>
<code>int</code>
<td><a name="84685"></a>
integer

<tr><td><a name="84689"></a>
J
<td><a name="84691"></a>
<code>long</code>
<td><a name="84693"></a>
long integer

<tr><td><a name="84697"></a>
L&lt;classname&gt;;
<td><a name="84699"></a>
<code>reference</code>
<td><a name="84701"></a>
an instance of class <code>&lt;classname&gt;</code>

<tr><td><a name="84705"></a>
S
<td><a name="84707"></a>
<code>short</code>
<td><a name="84709"></a>
signed short

<tr><td><a name="84713"></a>
Z
<td><a name="84715"></a>
<code>boolean</code>
<td><a name="84717"></a>
<code>true</code> or <code>false</code>

<tr><td><a name="84783"></a>
<strong>[&#32;</strong>
<td><a name="84785"></a>
<code>reference</code>
<td><a name="84787"></a>
one array dimension

</Table><br><br><p>
<a name="84641"></a>
<p>
<a name="84868"></a>
For example, the descriptor of an instance variable of type <code>int</code> is simply I. The descriptor of an instance variable of type <code>Object</code> is Ljava/lang/Object;. Note that the internal form of the fully qualified name for class <code>Object</code> is used. The descriptor of an instance variable that is a multidimensional <code>double</code> array,<p>
<pre>&nbsp;&nbsp;&nbsp;&nbsp;<code>double d[][][];
</code></pre><a name="12776"></a>
is
<p><pre>&nbsp;&nbsp;&nbsp;&nbsp;<strong>[[[D
</strong></pre><a name="7035"></a>
<h3>4.3.3    Method Descriptors</h3>
<a name="74607"></a>
A <i>method descriptor</i> represents the parameters that the method takes and the value 
that it returns:
<p><a name="81101"></a>
<blockquote>
<i>MethodDescriptor:</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;	( <i>ParameterDescriptor* &#32; &#32;</i>) <i>ReturnDescriptor</i><p>
<a name="81108"></a>
</blockquote>
A <i>parameter descriptor </i>represents a parameter passed to a method:<p>
<blockquote>
<a name="14034"></a>
<i>ParameterDescriptor:</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;	<i>FieldType</i><p>
<a name="42504"></a>
</blockquote>
A <i>return descriptor </i>represents the type of the value returned from a method. It is a series of characters generated by the grammar:<p>
<a name="14049"></a>
<blockquote>
<i>ReturnDescriptor:</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;	<i>FieldType</i><br>
	&nbsp;&nbsp;&nbsp;&nbsp;V<p>
	</blockquote>
<a name="7587"></a>
The character V indicates that the method returns no value (its return type is <code>void</code>).<p>
<a name="11151"></a>
A method descriptor is valid only if it represents method parameters with a total length of 255 or less, where that length includes the contribution for <code>this</code> in the case of instance or interface method invocations. The total length is calculated by summing the contributions of the individual parameters, where a parameter of type <code>long</code> or <code>double</code> contributes two units to the length and a parameter of any other type contributes one unit.<p>
<a name="12746"></a>
For example, the method descriptor for the method<p>
<pre>&nbsp;&nbsp;&nbsp;&nbsp;<code>Object mymethod(int i, double d, Thread t)
</code></pre><a name="12751"></a>
is
<p><pre>&nbsp;&nbsp;&nbsp;&nbsp;<strong>(IDLjava/lang/Thread;)Ljava/lang/Object;
</strong></pre><a name="18238"></a>
Note that internal forms of the fully qualified names of <code>Thread</code> and <code>Object</code> are 
used in the method descriptor.
<p><a name="15525"></a>
The method descriptor for <code>mymethod</code> is the same whether <code>mymethod</code> is a class or an instance method. Although an instance method is passed <code>this</code>, a reference to the current class instance, in addition to its intended parameters, that fact is not reflected in the method descriptor. (A reference to <code>this</code> is not passed to a class method.) The reference to <code>this</code> is passed implicitly by the method invocation instructions of the Java virtual machine used to invoke instance methods.<p>
<a name="20080"></a>
<hr><h2>4.4    The Constant Pool</h2>
<a name="9597"></a>
Java virtual machine instructions do not rely on the runtime layout of classes, 
interfaces, class instances, or arrays. Instead, instructions refer to symbolic information
in the <code>constant_pool</code> table. 
<p><a name="36992"></a>