How to force a class to be initialised? [closed]

What is the best and cleanest way to do this? Specifically, I need some code in a static initializer block to run in that class, but I'd like to make this as clean-looking as possible.

Loading != Initialization.

You want your class to be initialized (this is when static blocks executed, among other things).

An excerpt from the Java Language Specification says:

A class or interface type T will be initialized immediately before the first occurrence of >any one of the following:

T is a class and an instance of T is created.

T is a class and a static method declared by T is invoked.

A static field declared by T is assigned.

A static field declared by T is used and the field is not a constant variable (§4.12.4).

T is a top-level class, and an assert statement (§14.10) lexically nested within T is executed.

Invocation of certain reflective methods in class Class and in package java.lang.reflect also causes class or interface initialization. A class or interface will not be initialized under any other circumstance.

Doh, anovstrup, already said it: Just make an empty static function called init. Be sure to document that well... I personally can't see any use case for this in the context of well formed code though.

You can use the following code to force initialization of a class:

//... Foo.class ...          //OLD CODE
... forceInit(Foo.class) ... //NEW CODE

/**
 * Forces the initialization of the class pertaining to 
 * the specified <tt>Class</tt> object. 
 * This method does nothing if the class is already
 * initialized prior to invocation.
 *
 * @param klass the class for which to force initialization
 * @return <tt>klass</tt>

 */
public static <T> Class<T> forceInit(Class<T> klass) {
    try {
        Class.forName(klass.getName(), true, klass.getClassLoader());
    } catch (ClassNotFoundException e) {
        throw new AssertionError(e);  // Can't happen
    }
    return klass;
}

try
{
  Class.forName(class name as string)
}
catch(ClassNotFoundException e)
{
  whatever
}

That should do it.

@Longpoke

Maybe I am misunderstanding something then. Can you create an example where a class is loaded but the static initializer is not executed? Here is an example that does nothing but print out that it has loaded:

package test;

public class TestStatic 
{
    public static void main(String[] args) 
    {
        try 
        {
            Class.forName("test.Static");
        }
        catch (ClassNotFoundException e) 
        {
            e.printStackTrace();
        }
    }
}

With the following Static class being loaded:

package test;

public class Static 
{
    static
    {
        System.out.println("Static Initializer ran...");
    }
}

If the static initializers are not ran until the conditions you listed are met then why does this println get executed when I run my test? That is which of your listed conditions am I meeting?

How to force a class to be initialised? [closed]

Related

Recent Posts