Will a static block execute without the main method?

Question

I'm trying to run the below code on my machine, but it didn't execute anything nor showing any errors.

public class StaticBlockDemo {     static {         System.out.println("Hello World");     } } 

Can someone please help me? By the way, I'm using Java 7.

Answer 1

If you put a System.exit(0) at the end of the static-block, it will run with no errors in Java 6 and below (without a valid main!). This is because the static block is executed before a valid main method is searched for, so if you exit the program at the end of the static block, you will receive no errors.

However, this behavior was changed in Java 7; now you must include an explicit main, even if it might never be reached.

In Java 7, the answer to the question is false, but in Java 6 and below the answer is indeed true.

---

public class Test {     static {         System.out.println("Hello World");         System.exit(0);     } } 

Java 6:

 Hello World 

Java 7:

 Error: Main method not found in class Test, please define the main method as:    public static void main(String[] args) 

Answer 2

Static blocks execute when a class is initialized. Normally, the main class will cause initialization of the bootstrap class, but there are other ways to bootstrap a program, for example via a VM's native embedding API.

Invoking the static main method of a class causes its initialization, but so do many other things:

1. creating an instance of that class,
2. calling any other static method,
3. reading a static field (that is not final or has type other than a primitive type or String).

For more detail see the JLS chapter 12.4

The below shows this in action

public class Foo {   static { System.out.println("Foo initialized"); }    public static void main(String... argv) {     Initialized.callingThisCausesClassInitialization();   }    static class Initialized {     static { System.out.println("Initialized initialized"); }     static void callingThisCausesClassInitialization() {}   }    static class NotInitialized {     static { System.out.println("NotInitialized initialized"); }     static void callingThisCausesClassInitialization() {}   } } 

Running foo will print

Foo initialized Initialized initialized 

It will not print

NotInitialized initialized 

because nothing is done during the execution of that program that causes its initialization.

It looks like your class has that behavior because it is never used, like NotInitialized above.