java - 是什么导致此错误 : java. lang.VerifyError : Inconsistent stack height 2 ! = 1？

Question

我使用 ASM 为 while() 语句生成字节码。但是 eclipse 报告:

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)
          ..................

我的字节码源码:

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

为上述代码生成的字节码:

/ 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
}

我的生成字节码的java代码(我认为这个错误是由while()语句引起的，所以我只贴出这部分):

/* 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

让我们手动分析您的字节码:

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

所以您在通往 L1 的两条路径上的堆栈深度不一致，导致您看到的验证错误。

在我看来，您的错误是 L0 和 L2 block 中无用的“GETSTATIC”字节码——您正在插入 的值在堆栈上标记，但从不对其进行任何操作。