[java] Static Initialization Blocks

As far as I understood the "static initialization block" is used to set values of static field if it cannot be done in one line.

But I do not understand why we need a special block for that. For example we declare a field as static (without a value assignment). And then write several lines of the code which generate and assign a value to the above declared static field.

Why do we need this lines in a special block like: static {...}?

It is a common misconception to think that a static block has only access to static fields. For this I would like to show below piece of code that I quite often use in real-life projects (copied partially from another answer in a slightly different context):

public enum Language { 
  ENGLISH("eng", "en", "en_GB", "en_US"),   
  GERMAN("de", "ge"),   
  CROATIAN("hr", "cro"),   
  RUSSIAN("ru"),
  BELGIAN("be",";-)");

  static final private Map<String,Language> ALIAS_MAP = new HashMap<String,Language>(); 
  static { 
    for (Language l:Language.values()) { 
      // ignoring the case by normalizing to uppercase
      ALIAS_MAP.put(l.name().toUpperCase(),l); 
      for (String alias:l.aliases) ALIAS_MAP.put(alias.toUpperCase(),l); 
    } 
  } 

  static public boolean has(String value) { 
    // ignoring the case by normalizing to uppercase
    return ALIAS_MAP.containsKey(value.toUpper()); 
  } 

  static public Language fromString(String value) { 
    if (value == null) throw new NullPointerException("alias null"); 
    Language l = ALIAS_MAP.get(value); 
    if (l == null) throw new IllegalArgumentException("Not an alias: "+value); 
    return l; 
  } 

  private List<String> aliases; 
  private Language(String... aliases) { 
    this.aliases = Arrays.asList(aliases); 
  } 
} 

Here the initializer is used to maintain an index (ALIAS_MAP), to map a set of aliases back to the original enum type. It is intended as an extension to the built-in valueOf method provided by the Enum itself.

As you can see, the static initializer accesses even the private field aliases. It is important to understand that the static block already has access to the Enum value instances (e.g. ENGLISH). This is because the order of initialization and execution in the case of Enum types, just as if the static private fields have been initialized with instances before the static blocks have been called:

1. The Enum constants which are implicit static fields. This requires the Enum constructor and instance blocks, and instance initialization to occur first as well.
2. static block and initialization of static fields in the order of occurrence.

This out-of-order initialization (constructor before static block) is important to note. It also happens when we initialize static fields with the instances similarly to a Singleton (simplifications made):

public class Foo {
  static { System.out.println("Static Block 1"); }
  public static final Foo FOO = new Foo();
  static { System.out.println("Static Block 2"); }
  public Foo() { System.out.println("Constructor"); }
  static public void main(String p[]) {
    System.out.println("In Main");
    new Foo();
  }
}

What we see is the following output:

Static Block 1
Constructor
Static Block 2
In Main
Constructor

Clear is that the static initialization actually can happen before the constructor, and even after:

Simply accessing Foo in the main method, causes the class to be loaded and the static initialization to start. But as part of the Static initialization we again call the constructors for the static fields, after which it resumes static initialization, and completes the constructor called from within the main method. Rather complex situation for which I hope that in normal coding we would not have to deal with.

For more info on this see the book "Effective Java".

If your static variables need to be set at runtime then a static {...} block is very helpful.

For example, if you need to set the static member to a value which is stored in a config file or database.

Also useful when you want to add values to a static Map member as you can't add these values in the initial member declaration.

It's also useful when you actually don't want to assign the value to anything, such as loading some class only once during runtime.

E.g.

static {
    try {
        Class.forName("com.example.jdbc.Driver");
    } catch (ClassNotFoundException e) {
        throw new ExceptionInInitializerError("Cannot load JDBC driver.", e);
    }
}

Hey, there's another benefit, you can use it to handle exceptions. Imagine that getStuff() here throws an Exception which really belongs in a catch block:

private static Object stuff = getStuff(); // Won't compile: unhandled exception.

then a static initializer is useful here. You can handle the exception there.

Another example is to do stuff afterwards which can't be done during assigning:

private static Properties config = new Properties();

static {
    try { 
        config.load(Thread.currentThread().getClassLoader().getResourceAsStream("config.properties");
    } catch (IOException e) {
        throw new ExceptionInInitializerError("Cannot load properties file.", e);
    }
}

To come back to the JDBC driver example, any decent JDBC driver itself also makes use of the static initializer to register itself in the DriverManager. Also see this and this answer.

So you have a static field (it's also called "class variable" because it belongs to the class rather than to an instance of the class; in other words it's associated with the class rather than with any object) and you want to initialize it. So if you do NOT want to create an instance of this class and you want to manipulate this static field, you can do it in three ways:

1- Just initialize it when you declare the variable:

static int x = 3;

2- Have a static initializing block:

static int x;

static {
 x=3;
}

3- Have a class method (static method) that accesses the class variable and initializes it: this is the alternative to the above static block; you can write a private static method:

public static int x=initializeX();

private static int initializeX(){
 return 3;
}

Now why would you use static initializing block instead of static methods?

It's really up to what you need in your program. But you have to know that static initializing block is called once and the only advantage of the class method is that they can be reused later if you need to reinitialize the class variable.

let's say you have a complex array in your program. You initialize it (using for loop for example) and then the values in this array will change throughout the program but then at some point you want to reinitialize it (go back to the initial value). In this case you can call the private static method. In case you do not need in your program to reinitialize the values, you can just use the static block and no need for a static method since you're not gonna use it later in the program.

Note: the static blocks are called in the order they appear in the code.

Example 1:

class A{
 public static int a =f();

// this is a static method
 private static int f(){
  return 3;
 }

// this is a static block
 static {
  a=5;
 }

 public static void main(String args[]) {
// As I mentioned, you do not need to create an instance of the class to use the class variable
  System.out.print(A.a); // this will print 5
 }

}

Example 2:

class A{
 static {
  a=5;
 }
 public static int a =f();

 private static int f(){
  return 3;
 }

 public static void main(String args[]) {
  System.out.print(A.a); // this will print 3
 }

}

You can execute bits of code once for a class before an object is constructed in the static blocks.

E.g.

class A {
  static int var1 = 6;
  static int var2 = 9;
  static int var3;
  static long var4;

  static Date date1;
  static Date date2;

  static {
    date1 = new Date();

    for(int cnt = 0; cnt < var2; cnt++){
      var3 += var1;
    }

    System.out.println("End first static init: " + new Date());
  }
}

static block is used for any technology to initialize static data member in dynamic way,or we can say for the dynamic initialization of static data member static block is being used..Because for non static data member initialization we have constructor but we do not have any place where we can dynamically initialize static data member

Eg:-class Solution{
         // static int x=10;
           static int x;
       static{
        try{
          x=System.out.println();
          }
         catch(Exception e){}
        }
       }

     class Solution1{
      public static void main(String a[]){
      System.out.println(Solution.x);
        }
        }

Now my static int x will initialize dynamically ..Bcoz when compiler will go to Solution.x it will load Solution Class and static block load at class loading time..So we can able to dynamically initialize that static data member..

}

static int B,H;
static boolean flag = true;
static{
    Scanner scan = new Scanner(System.in);
    B = scan.nextInt();
    scan.nextLine();
    H = scan.nextInt();

    if(B < 0 || H < 0){
        flag = false;
        System.out.println("java.lang.Exception: Breadth and height must be positive");
    } 
}

You first need to understand that your application classes themselves are instantiated to java.class.Class objects during runtime. This is when your static blocks are ran. So you can actually do this:

public class Main {

    private static int myInt;

    static {
        myInt = 1;
        System.out.println("myInt is 1");
    }

    //  needed only to run this class
    public static void main(String[] args) {
    }

}

and it would print "myInt is 1" to console. Note that I haven't instantiated any class.

Here's an example:

  private static final HashMap<String, String> MAP = new HashMap<String, String>();
  static {
    MAP.put("banana", "honey");
    MAP.put("peanut butter", "jelly");
    MAP.put("rice", "beans");
  }

The code in the "static" section(s) will be executed at class load time, before any instances of the class are constructed (and before any static methods are called from elsewhere). That way you can make sure that the class resources are all ready to use.

It's also possible to have non-static initializer blocks. Those act like extensions to the set of constructor methods defined for the class. They look just like static initializer blocks, except the keyword "static" is left off.

I would say static block is just syntactic sugar. There is nothing you could do with static block and not with anything else.

To re-use some examples posted here.

This piece of code could be re-written without using static initialiser.

Method #1: With static

private static final HashMap<String, String> MAP;
static {
    MAP.put("banana", "honey");
    MAP.put("peanut butter", "jelly");
    MAP.put("rice", "beans");
  }

Method #2: Without static

private static final HashMap<String, String> MAP = getMap();
private static HashMap<String, String> getMap()
{
    HashMap<String, String> ret = new HashMap<>();
    ret.put("banana", "honey");
    ret.put("peanut butter", "jelly");
    ret.put("rice", "beans");
    return ret;
}

As supplementary, like @Pointy said

The code in the "static" section(s) will be executed at class load time, before any instances of the class are constructed (and before any static methods are called from elsewhere).

It's supposed to add System.loadLibrary("I_am_native_library") into static block.

static{
    System.loadLibrary("I_am_a_library");
}

It will guarantee no native method be called before the related library is loaded into memory.

According to loadLibrary from oracle:

If this method is called more than once with the same library name, the second and subsequent calls are ignored.

So quite unexpectedly, putting System.loadLibrary is not used to avoid library be loaded multi-times.

If they weren't in a static initialization block, where would they be? How would you declare a variable which was only meant to be local for the purposes of initialization, and distinguish it from a field? For example, how would you want to write:

public class Foo {
    private static final int widgets;

    static {
        int first = Widgets.getFirstCount();
        int second = Widgets.getSecondCount();
        // Imagine more complex logic here which really used first/second
        widgets = first + second;
    }
}

If first and second weren't in a block, they'd look like fields. If they were in a block without static in front of it, that would count as an instance initialization block instead of a static initialization block, so it would be executed once per constructed instance rather than once in total.

Now in this particular case, you could use a static method instead:

public class Foo {
    private static final int widgets = getWidgets();

    static int getWidgets() {
        int first = Widgets.getFirstCount();
        int second = Widgets.getSecondCount();
        // Imagine more complex logic here which really used first/second
        return first + second;
    }
}

... but that doesn't work when there are multiple variables you wish to assign within the same block, or none (e.g. if you just want to log something - or maybe initialize a native library).

There are a few actual reasons that it is required to exist:

1. initializing static final members whose initialization might throw an exception
2. initializing static final members with calculated values

People tend to use static {} blocks as a convenient way to initialize things that the class depends on within the runtime as well - such as ensuring that particular class is loaded (e.g., JDBC drivers). That can be done in other ways; however, the two things that I mention above can only be done with a construct like the static {} block.