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

Question

I use ASM to generate byte code for while() statement. 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 above code:

/ 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 while() statement, so I just post 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;
    }

Answer 1

Lets 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

So you have an inconsistent stack depth on two paths to L1, causing the verify error you see.

It seems to me, your error is the useless 'GETSTATIC' bytecodes in the blocks at L0 and L2 -- you're pushing the value of flag on the stack, but never doing anything with it.

Answer 2

This is because of the built in tomcat compiler.

The Java compiler from Eclipse JDT in included as the default compiler. It is an advanced Java compiler which will load all dependencies from the Tomcat class loader, which will help tremendously when compiling on large installations with tens of JARs. On fast servers, this will allow sub-second recompilation cycles for even large JSP pages.

If any such issues is seen follow the below workaround

Apache Ant, which was used in previous Tomcat releases, can be used instead of the new compiler by simply removing the lib/ecj-*.jar file, and placing the ant.jar and ant-launcher.jar files from the latest Ant distribution in the lib folder.