Variable 'final' modifier lost in Bytecode?

问题

Analyzing the bytecode of this simple class, I have come to the conclusion that the compiler doesn't retain any information about a local variable being final. This seems weird though, since I believe the HotSpot compiler could actually use this information to do optimizations.

Code:

public static void main(String[] args)
{
    final int i = 10;
    System.out.println(i);
}

Bytecode:

public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
  stack=2, locals=2, args_size=1
     0: bipush        10
     2: istore_1      
     3: getstatic     #16                 // Field java/lang/System.out:Ljava/io/PrintStream;
     6: bipush        10
     8: invokevirtual #22                 // Method java/io/PrintStream.println:(I)V
    11: return        
  LineNumberTable:
    line 7: 0
    line 8: 3
    line 9: 11
  LocalVariableTable:
    Start  Length  Slot  Name   Signature
        0      12     0  args   [Ljava/lang/String;
        3       9     1     i   I

Is there any specific reason not to retain the access flags of a local variable, other than saving disk space? Because to me, it seems that being final is a relatively non-trivial property of a variable.

回答1:

The final modifier is not present in the bytecode but the compiler already uses this information to make some optimization. Although your example doesn't show it, the compiler may inline the final variable's value in the bytecode representation of the method, leading to a better performance. Something like the below can show the difference:

public int addFinal() {
    final int i = 10;
    final int j = 10;
    return i + j;
}

public int addNonFinal() {
    int i = 10;
    int j = 10;
    return i + j;
}

The generated bytecode are respectively for each method:

// addFinal
bipush 10
istore_1
bipush 10
istore_2
bipush 20
ireturn


// addNonFinal
bipush 10
istore_1
bipush 10
istore_2
iload_1
iload_2
iadd
ireturn

来源：https://stackoverflow.com/questions/30151295/variable-final-modifier-lost-in-bytecode

标签

java

jvm

final

jvm-bytecode