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POHow to create a local variable with ASM?
 

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  1. POHow to create a local variable with ASM?
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    <p>I'm trying to patch a class with ASM. I need to add some logic in a function. This logic needs a new local variable. Here is what I've done:</p> <pre><code>class CreateHashTableMethodAdapter extends MethodAdapter { @Override public void visitMethodInsn(int opcode, String owner,String name, String desc){ System.out.println(opcode + "/" + owner + "/" + name + "/" + desc); if(opcode == Opcodes.INVOKESPECIAL &amp;&amp; "javax/naming/InitialContext".equals(owner) &amp;&amp; "&lt;init&gt;".equals(name) &amp;&amp; "()V".equals(desc)){ System.out.println("In mod"); // 83: new #436; //class javax/naming/InitialContext // 86: dup mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "javax/naming/InitialContext", "&lt;init&gt;", "()V"); mv.visitVarInsn(Opcodes.ASTORE, 1); Label start_patch = new Label(); Label end_patch = new Label(); mv.visitLabel(start_patch); mv.visitTypeInsn(Opcodes.NEW,"java/util/Hashtable"); mv.visitInsn(Opcodes.DUP); mv.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/util/Hashtable", "&lt;init&gt;", "()V"); mv.visitVarInsn(Opcodes.ASTORE,9); // ........ sNip .......... mv.visitLabel(end_patch); mv.visitLocalVariable("env","Ljava/util/Hashtable;",null,start_patch,end_patch,9); // 127: astore_1 } else { mv.visitMethodInsn(opcode, owner, name, desc); } } } </code></pre> <p>When I run this method adapter against <code>CheckClassAdapter</code> it states:</p> <pre><code>org.objectweb.asm.tree.analysis.AnalyzerException: Error at instruction 51: Trying to access an inexistant local variable 9 .... sNiP .... 00050 R R . . . : R R : INVOKESPECIAL java/util/Hashtable.&lt;init&gt; ()V 00051 R R . . . : R : ASTORE 9 </code></pre> <p>I think I misuse the visitLocalVariable, but I can not find out where I'm supposed to call it.</p> <p>When I javap generated bytecode (without checking), I get the following local variables table:</p> <pre><code>LocalVariableTable: Start Length Slot Name Signature 91 40 9 env Ljava/util/Hashtable; 0 343 0 this Lpmu/jms/ServerJMS; 132 146 1 initialContext Ljavax/naming/InitialContext; 153 125 2 topicConnectionFactory Ljavax/jms/TopicConnectionFactory; 223 55 3 topic Ljavax/jms/Topic; 249 29 4 topicSubscriber Ljavax/jms/TopicSubscriber; 279 55 1 ex Ljava/lang/Exception; 281 53 2 codeMessage I 289 45 3 params Lpmu/data/Parameters; 325 9 4 messageError Ljava/lang/String; </code></pre> <p>As you may notice, my variable is here but topmost ?! Any idea ?</p>
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    1. PO
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      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJens Björnhager
    1. USJérôme
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
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    1. PO
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    1. This table or related slice is empty.
    1. COThe visitLocalVariable call is only needed if you want to generate debug information. Otherwise it can be removed. The exception you posted is from data flow analyzer, which is not looking into debug info.
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      1. POHow to create a local variable with ASM?
      1. USEugene Kuleshov
    2. COAre you passing ClassWriter.COMPUTE_MAXS when constructing your ClassWriter, and / or calling `visitMaxs`? You'll need to do that to change the maximum size of the local variable table.
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      1. POHow to create a local variable with ASM?
      1. USint3
    3. CO@EugeneKuleshov : OK I didn't know that variables declaration are not needed. So, can I assume that if I want to use a local variable I just need the table to be big enough?
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      1. POHow to create a local variable with ASM?
      1. USJérôme
 

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