JLS 是否需要内联最终字符串常量？

Question

我运行在处理一些字节码时遇到问题，其中某个 final String 常量未被 java 编译器内联（Java 8), 见下例:

public class MyTest
{
  private static final String ENABLED  = "Y";
  private static final String DISABLED = "N";

  private static boolean isEnabled(String key) {
      return key.equals("A");
  }

  private static String getString(String key, String value) {
      return key + value;
  }

  public static void main(String[] args) throws Exception {
    String flag = getString("F", isEnabled("A") ? ENABLED : DISABLED);
    System.out.println(flag);

    String flag2 = getString("F", isEnabled("A") ? ENABLED : DISABLED);
    System.out.println(flag2);
  }
}

使用 javac (1.8.0_101)

生成的字节码

public static void main(java.lang.String[]) throws java.lang.Exception;
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: ldc           #8                  // String F
         2: ldc           #2                  // String A
         4: invokestatic  #9                  // Method isEnabled:(Ljava/lang/String;)Z
         7: ifeq          16
        10: getstatic     #10                 // Field ENABLED:Ljava/lang/String;
        13: goto          19
        16: getstatic     #11                 // Field DISABLED:Ljava/lang/String;
        19: invokestatic  #12                 // Method getString:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
        22: astore_1
        23: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
        26: aload_1
        27: invokevirtual #14                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        30: ldc           #8                  // String F
        32: ldc           #2                  // String A
        34: invokestatic  #9                  // Method isEnabled:(Ljava/lang/String;)Z
        37: ifeq          46
        40: getstatic     #10                 // Field ENABLED:Ljava/lang/String;
        43: goto          49
        46: getstatic     #11                 // Field DISABLED:Ljava/lang/String;
        49: invokestatic  #12                 // Method getString:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
        52: astore_2
        53: getstatic     #13                 // Field java/lang/System.out:Ljava/io/PrintStream;
        56: aload_2
        57: invokevirtual #14                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        60: return

可以看到第二次访问字段ENABLED和DISABLED时，编译器没有内联它们的值（使用ldc），而是使用getstatic 直接访问字段。使用其他编译器（Java 7，Eclipse）对其进行测试并没有触发相同的行为，常量总是内联的。

这可以被认为是编译器错误，还是允许不根据 JLS 始终内联字符串常量？

Answer 1

是的，“内联”行为是规范强制要求的：

13.1. The Form of a Binary

…

A reference to a field that is a constant variable (§4.12.4) must be resolved at compile time to the value V denoted by the constant variable's initializer.

If such a field is static, then no reference to the field should be present in the code in a binary file, including the class or interface which declared the field. Such a field must always appear to have been initialized (§12.4.2); the default initial value for the field (if different than V) must never be observed.

If such a field is non-static, then no reference to the field should be present in the code in a binary file, except in the class containing the field. (It will be a class rather than an interface, since an interface has only static fields.) The class should have code to set the field's value to V during instance creation (§12.5).

请注意，这如何精确地解决您的情况：“如果这样的字段是 static，那么二进制文件中的代码中不应存在对该字段的引用，包括class 或声明字段的接口”。

换句话说，如果你遇到一个不遵守这个的编译器，你发现了一个编译器错误。

作为补充，查找此信息的起点是：

4.12.4. final Variables

…

A constant variable is a final variable of primitive type or type String that is initialized with a constant expression (§15.28). Whether a variable is a constant variable or not may have implications with respect to class initialization (§12.4.1), binary compatibility (§13.1, §13.4.9), and definite assignment (§16 (Definite Assignment)).

JLS 是否需要内联最终字符串常量？

Does the JLS require inlining of final String constants?

java

java-8

language-lawyer

javac

jls

13.1. The Form of a Binary

4.12.4. final Variables