I know how a static init block works.
Can anyone please tell me some typical uses of it.
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7 Answers
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.
2 Comments
Mykola Golubyev
If which class do you use your example with "security.Init.init()"?
Bozho
It is required by the API. See santuario.apache.org/Java/api/org/apache/xml/security/Init.html
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 to @Bozho for examples.
Do not do
public static Configuration configuration;
static {
confuguration = new Configuration();
configuration.setSomething(..);
configuration.setSomethingElse(..);
...
}
Use instead
public static Configuration configuration = createConfiguration();
or
public static Configuration configuration = YourConfiguration.create();
1 Comment
gpampara
Agreed. Use a block as stated above creates serious issues with unit testing.
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");
}
}
Comments
- Initializing a collection static field, like Map, List, Set, etc
- Initializing setter-based object which is also static
Comments
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.
answered Feb 3, 2010 at 20:00
Pascal Thivent
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Comments
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
Comments
- static block :- when we want to execute code at the time of loading of class then we can put the code in static block...
- init :- when we want to execute code at the time of initiated of object of a class then we can put the code in init block....
