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What causes this error: java.lang.VerifyError: inconsistent stack height 2! = 1?

I use ASM to generate byte code for while (). But eclipse reports:

Exception in thread "main" java.lang.VerifyError: (class: show_cise_image, method: main signature: ([Ljava/lang/String;)V) Inconsistent stack height 2 != 1
    at java.lang.Class.getDeclaredMethods0(Native Method)
          ..................

My source code for byte code:

show_cise_image {
    boolean flag;
    flag =  true;
    while(flag){
        flag = false;   
    }    
}

generated byte code for code above:

/ class version 51.0 (51)
// access flags 0x21
public class show_cise_image {


  // access flags 0x8
  static int v = 0

  // access flags 0x8
  static boolean flag = 0

  // access flags 0x9
  public static main(String[]) : void
   L0
    LINENUMBER 6 L0
    GETSTATIC show_cise_image.flag : boolean
    LDC 1
    PUTSTATIC show_cise_image.flag : boolean
    GOTO L1
   L2
    GETSTATIC show_cise_image.flag : boolean
    LDC 0
    PUTSTATIC show_cise_image.flag : boolean
   L1
    GETSTATIC show_cise_image.flag : boolean
    IFNE L2
    RETURN
   L3
    LOCALVARIABLE args String[] L0 L3 2
    LOCALVARIABLE x int L0 L3 0
    LOCALVARIABLE y int L0 L3 1
    MAXSTACK = 3
    MAXLOCALS = 3
}

my java code to generate byte code (I think this error is caused by the while () operation, so I will just send this part):

/* while(Expr){ stmt*}  */

@Override
    public Object visitIterationStmt(IterationStmt iterationStmt, Object arg)
            throws Exception {
        MethodVisitor mv = (MethodVisitor)arg;
        Label guardLabel = new Label();
        Label bodyLabel = new Label();
        mv.visitJumpInsn(GOTO, guardLabel);

        mv.visitLabel(bodyLabel);
        for(Stmt t : iterationStmt.stmtList)
            t.visit(this, mv);   // execute statements in body

        mv.visitLabel(guardLabel);
        iterationStmt.expr.visit(this, mv);  // put the result of expr on stack
        mv.visitJumpInsn(IFNE, bodyLabel);
        return null;
    }
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2 answers

Let's analyze your bytecode manually:

L0                     ; on entry stack is empty
  LINENUMBER 6 L0
  GETSTATIC show_cise_image.flag : boolean ; pushes a value, stack height is 1
  LDC 1                                    ; pushes a value, stack heighe is 2  
  PUTSTATIC show_cise_image.flag : boolean ; pop 1 value, stack height is 1
  GOTO L1           ; stack height 1 on going to L1...

L1                  ; stack height 1 from previous goto
  GETSTATIC show_cise_image.flag : boolean ; pushes a value, stack height is 2
  IFNE L2           ; pops 1 value for test, stack height 1 on branch

L2                  ; stack height 1 from previous branch
  GETSTATIC show_cise_image.flag : boolean ; pushes a value, stack height is 2
  LDC 0                                    ; pushes a value, stack height is 3
  PUTSTATIC show_cise_image.flag : boolean ; pops a value, stack height is 2
  ; fall through to L1 with stack height 2

Thus, you have an inconsistent stack depth on two paths up L1, causing a validation error.

, - - GETSTATIC L0 L2 - flag , .

+6

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Java Eclipse JDT . Java, Tomcat, JAR. JSP.

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