Java类工作流程[重复]

Question

如果我有一个带有常量的类，例如 AppConstants 并使用 public static final 变量从那里像 AppConstants.MY_STRING 它会一直重新创建这个类，或者它在运行时创建它，因为类中的字段是静态的？那么在常量中定义文件还是在“原地”定义文件更好？

Answer 1

类由类加载器加载一次，通常在需要时¹。当一个类被加载时，它的所有static 成员也会被加载并“与类一起存在”，而不是与该类的特定对象引用一起加载。多次使用类的static 字段不会触发类的重新加载。重新加载类的唯一方法是使用自定义类加载器。

在常量中定义文件还是“在原地”更好？

将它们定义为类中的static final 字段或使用enums。

---

¹定义当需要时：

如果您像这样声明原始或String 常量：

public class AppConstants {
    public static final int ONE = 1;
    public static final int TWO = 2;
    public static final int TEN = 10;
}

public class ClientTest {
    public static void main(String[] args) {
        System.out.println(AppConstants.ONE);
        System.out.println(AppConstants.TWO);
        System.out.println(AppConstants.TEN);
    }
}

当编译两个类并执行ClientTest 时，AppConstants 类将不会被加载（使用 HotSpot 测试），因为编译器将内联常量。为了评估结果，请执行以下操作：

> javac -cp:. AppConstants.java ClientTest.java
> javap -c ClientTest //you can see the generated bytecode
> java -verbose:class ClientTest //shows you the classes loaded to execute this app

你会看到这个。

来自javap -c ClientTest（正确代码）：

public static void main(java.lang.String[]);
  Code:
   0:   getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   iconst_1 //My Comment: constant with value 1
   4:   invokevirtual   #3; //Method java/io/PrintStream.println:(I)V
   7:   getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   10:  iconst_2 //My Comment: constant with value 2
   11:  invokevirtual   #3; //Method java/io/PrintStream.println:(I)V
   14:  getstatic   #2; //Field java/lang/System.out:Ljava/io/PrintStream;
   17:  bipush  10 //My Comment: constant with value 10
   19:  invokevirtual   #3; //Method java/io/PrintStream.println:(I)V
   22:  return

}

来自java -verbose:class ClientTest：

[Opened C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Object from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
// Lots of classes from rt.jar
[Loaded java.security.UnresolvedPermission from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.security.BasicPermissionCollection from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded ClientTest from file:/D:/tmp/] //<-- Class being executed
[Loaded sun.launcher.LauncherHelper$FXHelper from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Class$MethodArray from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Void from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
1
2
10
[Loaded java.lang.Shutdown from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Shutdown$Lock from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]

没有AppConstants 类被加载的痕迹，只有ClientTest。

但是如果你添加一个引用对象引用的常量，例如List，然后将加载该类。从上面的例子：

public class AppConstants {
    public static final int ONE = 1;
    public static final int TWO = 2;
    public static final int TEN = 10;
    public static final List<String> NAMES = Collections.unmodifiableList(Arrays.asList("Luiggi", "Andy"));
}

public class ClientTest {
    public static void main(String[] args) {
        System.out.println(AppConstants.ONE);
        System.out.println(AppConstants.TWO);
        System.out.println(AppConstants.TEN);
        System.out.println(Foo.NAMES);
    }
}

现在提供java -verbose:class ClientTest的相关输出：

//...
[Loaded ClientTest from file:/D:/tmp/] //our class being executed
[Loaded sun.launcher.LauncherHelper$FXHelper from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Class$MethodArray from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.lang.Void from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
1  //output of constants
2
10
[Loaded AppConstants from file:/D:/tmp/] //loads AppConstants class here because it's needed
[Loaded java.util.Arrays$ArrayList from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Loaded java.util.AbstractList$Itr from C:\Program Files\Java\jre1.8.0_51\lib\rt.jar]
[Luiggi, Andy] //output of AppConstants#NAMES