Static Initialization Blocks

Question

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

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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 lt String  String gt  MAP  static       MAP put  banana    honey        MAP put  peanut butter    jelly        MAP put  rice    beans          Method  2  Without static  private static final HashMap lt String  String gt  MAP   getMap    private static HashMap lt String  String gt  getMap         HashMap lt String  String gt  ret   new HashMap lt  gt         ret put  banana    honey        ret put  peanut butter    jelly        ret put  rice    beans        return ret

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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  lt  var2  cnt           var3    var1             System out println  End first static init      new Date

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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 lt String Language gt  ALIAS MAP   new HashMap lt String Language gt        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 lt String gt  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    The Enum constants which are implicit static fields  This requires the Enum constructor and instance blocks  and instance initialization to occur first as well  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

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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

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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

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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

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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

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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

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There are a few actual reasons that it is required to exist    initializing static final members whose initialization might throw an exception 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

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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

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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

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Here s an example     private static final HashMap lt String  String gt  MAP   new HashMap lt String  String gt       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

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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  lt  0    H  lt  0           flag   false          System out println  java lang Exception  Breadth and height must be positive

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The non-static block            Do Something        Gets called every time an instance of the class is constructed  The static block only gets called once  when the class itself is initialized  no matter how many objects of that type you create   Example   public class Test        static          System out println  Static                         System out println  Non-static block               public static void main String   args            Test t   new Test            Test t2   new Test              This prints   Static Non-static block Non-static block

[java] Static Initialization Blocks

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