Static Block

1. Introduction

In Java, a static block (also called a static initialization block) is a special block of code that runs automatically when the class is loaded into the JVM, before any object is created and even before the main() method runs.

Static blocks are used to:

• Initialize static variables
• Perform class-level setup
• Load configurations
• Execute code only once for the entire program

Understanding static blocks is essential for mastering class loading, static initialization, and JVM behavior.

2. What Is a Static Block?

A static block is declared using the static keyword:

static {
    // code here
}

Key Characteristics

• Executes only once when the class is loaded
• Runs before main()
• Used for static initialization
• Executes in the order they appear if multiple static blocks exist

Example

class Demo {
    static {
        System.out.println("Static Block Executed");
    }

    public static void main(String[] args) {
        System.out.println("Main Method Executed");
    }
}

Output:

Static Block Executed
Main Method Executed

Static block runs first because class loading happens before main().

3. Why Use Static Blocks?

Static blocks are useful when you need:

3.1 Complex Initialization of Static Variables

Example:

static int data;

static {
    data = 10 * 5;  // complex calculation
}

3.2 Loading Configuration Files

Example:

static Properties config = new Properties();

static {
    config.load(new FileInputStream("config.properties"));
}

3.3 Initializing static final variables

static final int MAX;

static {
    MAX = 100;  // allowed for final variables
}

3.4 Calling static helper methods during class load

4. Multiple Static Blocks

Java allows multiple static blocks. Execution happens top to bottom, in the order they appear.

class Test {
    static {
        System.out.println("Block 1");
    }

    static {
        System.out.println("Block 2");
    }
}

Output:

Block 1
Block 2

5. Static Block vs Static Method

Static blocks are intended for one-time setup only.

6. Static Block with Object Creation

Even if no objects are created, static blocks still run.

class A {
    static {
        System.out.println("Static Block of A");
    }
}

public class Main {
    public static void main(String[] args) {
        // No object created
    }
}

Output:

Static Block of A

Because class A was loaded.

7. Static Block and Class.forName()

Static blocks also run when the class is loaded dynamically using Class.forName().

Class.forName("Test");

This causes the static block inside Test to execute immediately.

8. Using Static Blocks to Load Native Libraries

static {
    System.loadLibrary("mylib");
}

This ensures the library is loaded only once.

9. Common Mistakes

Mistake 1: Trying to access instance variables

Static block cannot access non-static variables:

int x = 10;

static {
    System.out.println(x); // ERROR
}

Mistake 2: Using static blocks unnecessarily

If simple assignment works, no need for a static block.

Mistake 3: Expecting static block to run multiple times

It runs only once per class loading.

10. Complete Example Demonstrating Static Block Behavior

class DatabaseConnection {

    static String url;
    static String username;
    static String password;

    static {
        System.out.println("Loading configuration...");
        url = "jdbc:mysql://localhost/test";
        username = "root";
        password = "1234";
    }

    static void connect() {
        System.out.println("Connecting to " + url);
    }
}

public class Main {
    public static void main(String[] args) {
        DatabaseConnection.connect();
    }
}

Output:

Loading configuration...
Connecting to jdbc:mysql://localhost/test

11. Summary

• Static blocks execute once when the class loads.
• They run before main() and before any object creation.
• Used for initializing static variables and configurations.
• Multiple static blocks execute in order.
• Cannot access instance members.
• Useful for complex initialization and setup tasks.

This completes Static Block in Java.

Written By: Shiva Srivastava