What are the two ways of Java bytecode execution?

1. Description

(1) Just-in-time compilation method: The interpreter first compiles the bytecode into machine code and then executes the machine code.

(2) Interpretation and execution method: The interpreter completes all operations of the Java bytecode program by interpreting and executing a small piece of code each time.

The second method is commonly used. Because the JVM specification is flexible enough, it can convert bytecode into machine code more efficiently. For applications that have higher requirements for running speed, the interpreter can be used to immediately compile Java bytecode into machine code, thus ensuring the portability and high performance of Java code.

2. Example

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=4, args_size=1
         0: iconst_1
         1: istore_1
         2: iconst_2
         3: istore_2
         4: iconst_1
         5: iconst_2
         6: invokestatic  #2                  // Method calc:(II)I
         9: istore_3
        10: return
        
  static int calc(int, int);
    descriptor: (II)I
    flags: ACC_STATIC
    Code:
      stack=6, locals=2, args_size=2
         0: iload_0
         1: i2d
         2: ldc2_w        #3                  // double 2.0d
         5: invokestatic  #5                  // Method java/lang/Math.pow:(DD)D
         8: iload_1
         9: i2d
        10: ldc2_w        #3                  // double 2.0d
        13: invokestatic  #5                  // Method java/lang/Math.pow:(DD)D
        16: dadd
        17: invokestatic  #6                  // Method java/lang/Math.sqrt:(D)D
        20: d2i
        21: ireturn

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