Difference between static modifier and static bloc

Question

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• What is the difference between a static and a non-static initialization code block 8 answers

Someone explain to me the differences between the following two statements?

A static final variable initialized by a static code block:

private static final String foo;
static { foo = "foo"; }

A static final variable initialized by an assignment:

private static final String foo = "foo";

Answer 1

Static block give you more than simple statement. In this particular case is the same thing. Static section will be executed at class load time, before any instances constructed. You can call methods here and assign their results to static fields. And you can catch exceptions in static blocks.

Answer 2

The JLS describes a few special behaviors of what it calls constant variables, which are final variables (whether static or not) which are initialized with constant expressions of String or primitive type.

Constant variables have a major difference with respect to binary compatibility: the values of constant variables become part of the class's API, as far as the compiler is concerned.

An example:

class X {
    public static final String XFOO = "xfoo";
}

class Y {
    public static final String YFOO;
    static { YFOO = "yfoo"; }
}

class Z {
    public static void main(String[] args) {
        System.out.println(X.XFOO);
        System.out.println(Y.YFOO);
    }
}

Here, XFOO is a "constant variable" and YFOO is not, but they are otherwise equivalent. Class Z prints out each of them. Compile those classes, then disassemble them with javap -v X Y Z, and here is the output:

Class X:

Constant pool:
   #1 = Methodref          #3.#11         //  java/lang/Object."<init>":()V
   #2 = Class              #12            //  X
   #3 = Class              #13            //  java/lang/Object
   #4 = Utf8               XFOO
   #5 = Utf8               Ljava/lang/String;
   #6 = Utf8               ConstantValue
   #7 = String             #14            //  xfoo
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = NameAndType        #8:#9          //  "<init>":()V
  #12 = Utf8               X
  #13 = Utf8               java/lang/Object
  #14 = Utf8               xfoo
{
  public static final java.lang.String XFOO;
    descriptor: Ljava/lang/String;
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
    ConstantValue: String xfoo


  X();
    descriptor: ()V
    flags:
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
}

Class Y:

Constant pool:
   #1 = Methodref          #5.#12         //  java/lang/Object."<init>":()V
   #2 = String             #13            //  yfoo
   #3 = Fieldref           #4.#14         //  Y.YFOO:Ljava/lang/String;
   #4 = Class              #15            //  Y
   #5 = Class              #16            //  java/lang/Object
   #6 = Utf8               YFOO
   #7 = Utf8               Ljava/lang/String;
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               <clinit>
  #12 = NameAndType        #8:#9          //  "<init>":()V
  #13 = Utf8               yfoo
  #14 = NameAndType        #6:#7          //  YFOO:Ljava/lang/String;
  #15 = Utf8               Y
  #16 = Utf8               java/lang/Object
{
  public static final java.lang.String YFOO;
    descriptor: Ljava/lang/String;
    flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL


  Y();
    descriptor: ()V
    flags:
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return

  static {};
    descriptor: ()V
    flags: ACC_STATIC
    Code:
      stack=1, locals=0, args_size=0
         0: ldc           #2                  // String yfoo
         2: putstatic     #3                  // Field YFOO:Ljava/lang/String;
         5: return
}

Class Z:

Constant pool:
   #1 = Methodref          #8.#14         //  java/lang/Object."<init>":()V
   #2 = Fieldref           #15.#16        //  java/lang/System.out:Ljava/io/PrintStream;
   #3 = Class              #17            //  X
   #4 = String             #18            //  xfoo
   #5 = Methodref          #19.#20        //  java/io/PrintStream.println:(Ljava/lang/String;)V
   #6 = Fieldref           #21.#22        //  Y.YFOO:Ljava/lang/String;
   #7 = Class              #23            //  Z
   #8 = Class              #24            //  java/lang/Object
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               main
  #13 = Utf8               ([Ljava/lang/String;)V
  #14 = NameAndType        #9:#10         //  "<init>":()V
  #15 = Class              #25            //  java/lang/System
  #16 = NameAndType        #26:#27        //  out:Ljava/io/PrintStream;
  #17 = Utf8               X
  #18 = Utf8               xfoo
  #19 = Class              #28            //  java/io/PrintStream
  #20 = NameAndType        #29:#30        //  println:(Ljava/lang/String;)V
  #21 = Class              #31            //  Y
  #22 = NameAndType        #32:#33        //  YFOO:Ljava/lang/String;
  #23 = Utf8               Z
  #24 = Utf8               java/lang/Object
  #25 = Utf8               java/lang/System
  #26 = Utf8               out
  #27 = Utf8               Ljava/io/PrintStream;
  #28 = Utf8               java/io/PrintStream
  #29 = Utf8               println
  #30 = Utf8               (Ljava/lang/String;)V
  #31 = Utf8               Y
  #32 = Utf8               YFOO
  #33 = Utf8               Ljava/lang/String;
{
  Z();
    descriptor: ()V
    flags:
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #4                  // String xfoo
         5: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
        11: getstatic     #6                  // Field Y.YFOO:Ljava/lang/String;
        14: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        17: return
}

Things to notice in the disassembly, which tell you the differences between X and Y run deeper than syntactic sugar:

• XFOO has a ConstantValue attribute, signifying that its value is a compile-time constant. Whereas YFOO does not, and uses a static block with a putstatic instruction to initialize the value at runtime.

• The String constant "xfoo" has become part of class Z's constant pool, but "yfoo" has not.

• Z.main uses the ldc (load constant) instruction to load "xfoo" onto the stack directly from its own constant pool, but it uses a getstatic instruction to load the value of Y.YFOO.

Other differences you will find:

• If you change the value of XFOO and recompile X.java but not Z.java, you have a problem: class Z is still using the old value. If you change the value of YFOO and recompile Y.java, class Z uses the new value whether you recompile Z.java or not.

• If you delete the X.class file entirely, class Z still runs correctly. Z has no runtime dependency on X. Whereas if you delete the Y.class file, class Z fails to initialize with a ClassNotFoundException: Y.

• If you generate documentation for the classes with javadoc, the "Constant Field Values" page will document the value of XFOO, but not the value of YFOO.

The JLS describes the above effects constant variables have on compiled class files in §13.1.3:

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).

And in §13.4.9:

If a field is a constant variable (§4.12.4), and moreover is static, then deleting the keyword final or changing its value will not break compatibility with pre-existing binaries by causing them not to run, but they will not see any new value for a usage of the field unless they are recompiled.

[...]

The best way to avoid problems with "inconstant constants" in widely-distributed code is to use static constant variables only for values which truly are unlikely ever to change. Other than for true mathematical constants, we recommend that source code make very sparing use of static constant variables.

The upshot is that if your public library exposes any constant variables, you must never change their values if your new library version is otherwise supposed to be compatible with code compiled against old versions of the library. It won't necessarily cause an error, but the existing code will probably malfunction since it will have outdated ideas about the values of constants. (If your new library version needs for classes which use it to be recompiled anyway, then changing constants doesn't cause this problem.)

Thus, initializing a constant with a block gives you more freedom to change its value, because it prevents the compiler embedding the value into other classes.

Answer 3

In this example, there's one subtle difference - in your first example, foo isn't determined to be a compile-time constant, so it can't be used as a case in switch blocks (and wouldn't be inlined into other code); in your second example it, is. So for example:

switch (args[0]) {
    case foo:
        System.out.println("Yes");
        break;
}

That's valid when foo is deemed to be a constant expression, but not when it's "just" a static final variable.

However, static initializer blocks are usually used when you have more complicated initialization code - such as populating a collection.

The timing for initialization is described in JLS 12.4.2; any static final fields which are considered as compile-time constants are initialized first (step 6) and initializers are run later (step 9); all initializers (whether they're field initializers or static initializers) are run in textual order.

Answer 4

An additional aspect: Consider the case when you have multiple static fields, and yes this is a corner case...

As stated in Jon Skeet's answer, the JLS defines the exact order of initialization. However, if for some reason you have to initialize multiple static attributes in a specific order, you may want to make the initialization sequence clearly visible in the code. When using direct field initialization: Some code formatters (and developers) may decide at some point to sort fields differently, this will directly impact how the fields get initialized and introduce unwanted effects.

By the way, if you want to follow common java coding conventions, you should use capital letters when defining 'constants' (final static fields).

--- edited reflecting Jon Skeet's comments ---

Answer 5

The only difference is the initialization time.

Java first initializes the members and then the static blocks.

Answer 6

 private static final String foo;
 static { foo ="foo";}

The value of foo is initialized when the class is loaded and static initializers are run.

private static final String foo = "foo";

Here, the value of foo will be a compile-time constant. So, in reality "foo" will be available as part of th byte-code itself.