Use of static init block

Question

I know how a static init block works.
Can anyone please tell me some typical uses of it.

Answer 1

Just try to avoid use of static initialization block. Instead you can use private static initialization functions which will make your code more clean.

I will refer @Bozho examples.

Do not

public static Configuration configuration;
static {
     confuguration = new Configuration();
     configuration.setSomething(..);
     configuration.setSomethingElse(..);
     ...
}

Use

public static Configuration configuration = createConfiguration();

or

public static Configuration configuration = YourConfiguration.create();

Answer 2

When you want to initialize one or more static variables in one place

It is useful, because you can apply exception handling, which is not possible with the in-line initialization.

For example:

public static ImageIcon defaultIcon = ImageIO.read(..);

can be initialized with

public static ImageIcon defaultIcon;
static {
   try {
       defaultIcon = ImageIO.read(..);
   } catch (IOException ex){
     System.out.println("No default icon available");
   }
}

Another application is complex initialization. For example, if an item requires more than one line of code to be initialized. Let's say you have a configuration:

public static Configuration configuration;
static {
     confuguration = new Configuration();
     configuration.setSomething(..);
     configuration.setSomethingElse(..);
     ...
}

A third usage is to initialize some external API infrastructure. One example from my current project:

static {
    org.apache.xml.security.Init.init();
}

But, as Mykola Golubyev noted, static initialization blocks make code less readable, so use them with caution. static methods do the same thing more transparently.

Answer 3

• Initializing a collection static field, like Map, List, Set, etc
• Initializing setter-based object which is also static

Answer 4

They're often used in conjunction with JNI code to ensure that the required native library is loaded:

class MyJniConnection {

    public static native void myJniCall();

    static {
        System.load("native.dll");
    }
}

Answer 5

They can be used to create a DSL, as JMock does. For instance, to set an expectation that a user will be saved to the database:

Mockery context = new Mockery();
final Database database = context.mock(Database.class);    
...
context.checking(new Expectations() {{
    oneOf(database).save(user);
}});

// Rest of the test

Answer 6

JDBC Driver Is a Popular Example

Why do you need Class.forName() to load a driver into memory. The answer is simple. As stipulated in the JDBC specs, all JDBC Driver have a static block to register themselves with DriverManager as soon as the Driver class is loaded. Something like this:

static {
    try {
        java.sql.DriverManager.registerDriver(new Driver());
    } catch (SQLException E) {
        throw new RuntimeException("Can't register driver!");
    }
}

So, when you write (for example with the MySQL driver here):

Class.forName("org.gjt.mm.mysql.Driver");

The classloader attempts to load and link the org.gjt.mm.mysql.Driver class and, if successful, the static initialization block gets executed and the Driver registers itself with the DriverManager.