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ifle=158 branch1 branch2   Stack: ..., INT ->
...</TT>
</BLOCKQUOTE>
<H5>Comparison with Null</H5>
<BLOCKQUOTE>
<TT>ifnull=198 branch1 branch2&nbsp;&nbsp;&nbsp;&nbsp;Stack:
..., OBJ -&gt; ...<BR>
ifnonnull=199 branch1 branch2 Stack: ..., OBJ -&gt; ...</TT>
</BLOCKQUOTE>
<H5>Compare Two Integers</H5>
<BLOCKQUOTE>
<TT>if_icmpeq=159 branch1 branch2&nbsp;&nbsp;Stack:
..., INT1, INT2 -&gt; ...<BR>
if_icmpne=160 branch1 branch2&nbsp;&nbsp;Stack: ..., INT1, INT2
-&gt; ...<BR>
if_icmplt=161 branch1 branch2&nbsp;&nbsp;Stack: ..., INT1, INT2
-&gt; ...<BR>
if_icmpge=162 branch1 branch2&nbsp;&nbsp;Stack: ..., INT1, INT2
-&gt; ...<BR>
if_icmpgt=163 branch1 branch2&nbsp;&nbsp;Stack: ..., INT1, INT2
-&gt; ...<BR>
if_icmple=164 branch1 branch2&nbsp;&nbsp;Stack: ..., INT1, INT2
-&gt; ...</TT>
</BLOCKQUOTE>
<H5>Compare Two Long Integers</H5>
<BLOCKQUOTE>
<TT>lcmp=148  Stack: ..., L1_1, L1_2, L2_1,
L2_2 -&gt; ..., INT (One of [-1, 0, 1])</TT>
</BLOCKQUOTE>
<H5>Compare Two Floats </H5>
<P>
l-&gt;-1 on NaN, g-&gt;1 on NaN.
<BLOCKQUOTE>
<TT>fcmpl=149&nbsp;&nbsp;Stack: ..., FLOAT1,
FLOAT2 -&gt; ..., INT (One of [-1, 0, 1])<BR>
fcmpg=150&nbsp;&nbsp;Stack: ..., FLOAT1, FLOAT2 -&gt; ..., INT
(One of [-1, 0, 1])</TT>
</BLOCKQUOTE>
<H5>Compare Two Doubles </H5>
<P>
l-&gt;-1 on NaN, g-&gt;1 on NaN.
<BLOCKQUOTE>
<TT>dcmpl=151&nbsp;&nbsp;Stack: ..., D1_1,
D1_2, D2_1, D2_2 -&gt; ..., INT (One of [-1, 0, 1])<BR>
dcmpg=152&nbsp;&nbsp;Stack: ..., D1_1, D1_2, D2_1, D2_2 -&gt;
..., INT (One of [-1, 0, 1])</TT>
</BLOCKQUOTE>
<H5>Compare Two Objects</H5>
<BLOCKQUOTE>
<TT>if_acmpeq=165 branch1 branch2&nbsp;&nbsp;Stack:
..., OBJ1, OBJ2 -&gt; ...<BR>
if_acmpne=166 branch1 branch2&nbsp;&nbsp;Stack: ..., OBJ1, OBJ2
-&gt; ...</TT>
</BLOCKQUOTE>
<H5>Unconditional Branching</H5>
<P>
16-bit and 32-bit branching
<BLOCKQUOTE>
<TT>goto=167 branch1 branch2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Stack:
... -&gt; ...<BR>
goto_w=200 branch1 branch2 branch3 branch4&nbsp;&nbsp;Stack: ...
-&gt; ...</TT>
</BLOCKQUOTE>
<H5>Jump Subroutine </H5>
<P>
16-bit and 32-bit jumps
<BLOCKQUOTE>
<TT>jsr=168 branch1 branch2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Stack:
... -&gt; ..., returnAddress<BR>
jsr_w=201 branch1 branch2 branch3 branch4&nbsp;&nbsp;&nbsp;Stack:
... -&gt; ..., returnAddress</TT>
</BLOCKQUOTE>
<H5>Return from Subroutine </H5>
<P>
The return address is retrieved from a local variable, not the
stack.
<BLOCKQUOTE>
<TT>ret=169 vindex&nbsp;&nbsp;&nbsp;Stack:
... -&gt; ...&nbsp;&nbsp;&nbsp;(returnAddress &lt;- vars[vindex])
<BR>
ret_w=209 vindex1 vindex2&nbsp;&nbsp;&nbsp;Stack: ... -&gt; ...&nbsp;&nbsp;&nbsp;(returnAddress
&lt;- vars[vindex])</TT>
</BLOCKQUOTE>
<H5>Returning Primitives </H5>
<P>
The current stack frame is destroyed. The top primitive is pushed
onto the caller's operand stack.
<BLOCKQUOTE>
<TT>ireturn=172&nbsp;&nbsp;Stack: ..., INT
-&gt; [destroyed]<BR>
lreturn=173&nbsp;&nbsp;Stack: ..., LONG1, LONG2 -&gt; [destroyed]
<BR>
freturn=174&nbsp;&nbsp;Stack: ..., FLOAT -&gt; [destroyed]<BR>
dreturn=175&nbsp;&nbsp;Stack: ..., DOUBLE1, DOUBLE2 -&gt; [destroyed]
<BR>
areturn=176&nbsp;&nbsp;Stack: ..., OBJ -&gt; [destroyed]<BR>
return=177&nbsp;&nbsp;Stack: ... -&gt; [destroyed]</TT>
</BLOCKQUOTE>
<H5>Calling the Breakpoint Handler</H5>
<BLOCKQUOTE>
<TT>breakpoint=202&nbsp;&nbsp;Stack: ...,
-&gt; ...</TT>
</BLOCKQUOTE>
<H4>Manipulating Object Fields</H4>
<P>
Construct a 16-bit index into the constant pool to retrieve the
class and field name, then resolve these names to determine the
field offset and width. Use the object reference on the stack
as the target. The value will be 32 or 64 bits, depending on the
field information in the constant pool.
<BLOCKQUOTE>
<TT>Getstatic=178&nbsp;&nbsp;index1 index2&nbsp;&nbsp;&nbsp;Stack:
..., -&gt; ..., VAL<BR>
Putstatic=179&nbsp;&nbsp;index1 index2&nbsp;&nbsp;&nbsp;Stack:
..., VAL -&gt; ...<BR>
Getfield=180&nbsp;&nbsp;&nbsp;index1 index2&nbsp;&nbsp;&nbsp;Stack:
..., OBJ -&gt; ..., VAL<BR>
Putfield=181&nbsp;&nbsp;&nbsp;index1 index2&nbsp;&nbsp;&nbsp;Stack:
..., OBJ, VAL -&gt; ...</TT>
</BLOCKQUOTE>
<H4>Method Invocation</H4>
<P>
There are four types of method invocation:
<UL>
<LI><TT>invokevirtual=182</TT>-Normal
method dispatch in Java. Use the index bytes to create a 16-bit
index into the constant table of the current class. Extract the
method name and signature. Search the method table of the stack
object to determine the method address. Use the method signature
to remove the method arguments from the operand stack and transfer
them to the new method's local variables.
<LI><TT>invokenonvirtual=183</TT>-Used
when a method is called with the super keyword. Use the index
bytes to create a 16-bit index into the constant pool of the current
class. Extract the method name and signature. Search the named
class's method table to determine the method address. Extract
the object and arguments and place them in the new method's local
variables.
<LI><TT>invokestatic=184</TT>-Used
to call static methods. Create a 16-bit index into the current
class's constant pool. Extract the method and search the named
class's method table for the address. Transfer the arguments as
before. There is no object to pass.
<LI><TT>invokeinterface=185</TT>-Invoke
an interface function. Again, a 16-bit index is created to find
the method name and signature. This time, however, the number
of arguments is determined from the bytecodes, not the signature.
<BR>
<BR>
<TT>virtual index1 index2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Stack:
..., OBJ, [arg1, [arg2, ...]] -&gt; ...<BR>
nonvirtual index1 index2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
Stack: ..., OBJ, [arg1, [arg2, ...]] -&gt; ...<BR>
static index1 index2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Stack:
..., [arg1, [arg2, ...]] -&gt; ...<BR>
interface index1 index2 nargs resv Stack: ..., OBJ, [arg1, [arg2,
...]] -&gt; ...</TT>
</UL>
<H4>Miscellaneous Operations</H4>
<P>
These instructions don't fall under any other heading; they deal
with generic object operations, such as creation and casting.
<H5>Throw Exception</H5>
<BLOCKQUOTE>
<TT>athrow=191&nbsp;&nbsp;Stack: ..., OBJ
-&gt; [undefined]</TT>
</BLOCKQUOTE>
<H5>Create a New Object</H5>
<BLOCKQUOTE>
<TT>new=187 index1 index2&nbsp;&nbsp;Stack:
... -&gt; ..., OBJ</TT>
</BLOCKQUOTE>
<H5>Check a Cast Operation</H5>
<BLOCKQUOTE>
<TT>checkcast=192 index1 index2&nbsp;&nbsp;Stack:
..., OBJ -&gt; ..., OBJ</TT>
</BLOCKQUOTE>
<H5>Instanceof</H5>
<BLOCKQUOTE>
<TT>instanceof=193 index1 index2&nbsp;&nbsp;Stack:
..., OBJ -&gt; ... INT (1 or 0)</TT>
</BLOCKQUOTE>
<H4>Monitors</H4>
<P>
Monitor instructions are used for synchronization.
<H5>Enter a Monitored Region of Code</H5>
<BLOCKQUOTE>
<TT>monitorenter=194&nbsp;&nbsp;Stack: ...,
OBJ -&gt; ...</TT>
</BLOCKQUOTE>
<H5>Exit a Monitored Region of Code</H5>
<BLOCKQUOTE>
<TT>monitorexit=195&nbsp;&nbsp;Stack: ...,
OBJ -&gt; ...</TT>
</BLOCKQUOTE>
<H3><A NAME="TestClassBytecodes">Test Class Bytecodes</A></H3>
<P>
Sun supplies a tool, <TT>javap</TT>,
that enables you to disassemble and view the bytecodes of a class.
If the -c option is passed to <TT>javap</TT>,
a listing of bytecodes is produced. These are the test class's
bytecodes:
<BLOCKQUOTE>
<TT>Compiled from test.java<BR>
public class test extends java.lang.Object {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public static int st_one;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public test();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public test(int);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public native boolean getData(int []);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public int do_countdown();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;public int do_countdown(int);<BR>
<BR>
Method test()<BR>
&nbsp;&nbsp;&nbsp;0 aload_0<BR>
&nbsp;&nbsp;&nbsp;1 invokenonvirtual #9 &lt;Method java.lang.Object.&lt;init&gt;()V&gt;
<BR>
&nbsp;&nbsp;&nbsp;4 bipush 100<BR>
&nbsp;&nbsp;&nbsp;6 putstatic #7 &lt;Field test.st_one I&gt;<BR>
&nbsp;&nbsp;&nbsp;9 return<BR>
<BR>
Method test(int)<BR>
&nbsp;&nbsp;&nbsp;0 aload_0<BR>
&nbsp;&nbsp;&nbsp;1 invokenonvirtual #9 &lt;Method java.lang.Object.&lt;init&gt;()V&gt;
<BR>
&nbsp;&nbsp;&nbsp;4 iload_1<BR>
&nbsp;&nbsp;&nbsp;5 putstatic #7 &lt;Field test.st_one I&gt;<BR>
&nbsp;&nbsp;&nbsp;8 return<BR>
<BR>
Method int do_countdown()<BR>
&nbsp;&nbsp;&nbsp;0 getstatic #7 &lt;Field test.st_one I&gt;<BR>
&nbsp;&nbsp;&nbsp;3 istore_1<BR>
&nbsp;&nbsp;&nbsp;4 getstatic #6 &lt;Field java.lang.System.out
Ljava/io/PrintStream;&gt;<BR>
&nbsp;&nbsp;&nbsp;7 ldc #1 &lt;String &quot;Performing countdown:&quot;&gt;
<BR>
&nbsp;&nbsp;&nbsp;9 invokevirtual #10 &lt;Method java.io.PrintStream.println(Ljava/lang/String;)V&gt;
<BR>
&nbsp;&nbsp;12 goto 22<BR>
&nbsp;&nbsp;15 getstatic #6 &lt;Field java.lang.System.out Ljava/io/PrintStream;&gt;
<BR>
&nbsp;&nbsp;18 iload_1<BR>
&nbsp;&nbsp;19 invokevirtual #8 &lt;Method java.io.PrintStream.println(I)V&gt;
<BR>
&nbsp;&nbsp;22 iload_1<BR>
&nbsp;&nbsp;23 iinc 1 -1<BR>
&nbsp;&nbsp;26 ifne 15<BR>
&nbsp;&nbsp;29 getstatic #7 &lt;Field test.st_one I&gt;<BR>
&nbsp;&nbsp;32 ireturn<BR>
<BR>
Method int do_countdown(int)<BR>
&nbsp;&nbsp;&nbsp;0 iload_1<BR>
&nbsp;&nbsp;&nbsp;1 istore_2<BR>
&nbsp;&nbsp;&nbsp;2 getstatic #6 &lt;Field java.lang.System.out
Ljava/io/PrintStream;&gt;<BR>
&nbsp;&nbsp;&nbsp;5 ldc #1 &lt;String &quot;Performing countdown:&quot;&gt;
<BR>
&nbsp;&nbsp;&nbsp;7 invokevirtual #10 &lt;Method java.io.PrintStream.println(Ljava/lang/String;)V&gt;
<BR>
&nbsp;&nbsp;10 goto 20<BR>
&nbsp;&nbsp;13 getstatic #6 &lt;Field java.lang.System.out Ljava/io/PrintStream;&gt;
<BR>
&nbsp;&nbsp;16 iload_1<BR>
&nbsp;&nbsp;17 invokevirtual #8 &lt;Method java.io.PrintStream.println(I)V&gt;
<BR>
&nbsp;&nbsp;20 iload_1<BR>
&nbsp;&nbsp;21 iinc 1 -1<BR>
&nbsp;&nbsp;24 ifne 13<BR>
&nbsp;&nbsp;27 iload_2<BR>
&nbsp;&nbsp;28 ireturn<BR>
}</TT>
</BLOCKQUOTE>
<P>
The left-hand column displays the offset of the instruction. <TT>Javap</TT>
automatically converts jump displacements to actual offsets. In
addition, it looks up constant pool references in order to output
the corresponding strings.
<H2><A NAME="GarbageCollection"><FONT SIZE=5 COLOR=#FF0000>Garbage
Collection</FONT></A></H2>
<P>
Java uses a multitiered security mechanism. The bytecode verifier
provides the lowest layer of  security. Above the verifier, the
security manager is the next sentry. In addition to these two
explicit checks, there are a number of language features that
provide security as well. Chief among these is the garbage collector.
<P>
Failing to free memory blocks or file handles is a common bug
in most modern programs. The problem quickly escalates until the
system crashes in some unforeseen manner. Java, like Smalltalk
before it, uses implicit garbage collection to solve the problem.
The virtual machine spec does not mandate a particular type of
garbage collection; it requires only that some type be used.
<P>
In Sun's runtime, a mark and sweep algorithm is used. This enables
the garbage collector to run incrementally in the background.
<P>
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