When you have 200 concurrent users hitting a single-threaded service, you’ll see the connection pool exhaustion, leading to a java.sql.SQLException: Connection is closed error. This is because the default connection pool size is not sufficient to handle the load. To solve this, you need to increase the connection pool size or implement a more efficient connection pool strategy.
## Prerequisites To follow this tutorial, you need: * Java 17 * Spring Boot 2.7.3 * Maven 3.8.6 * The following Maven dependency:
<dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency>
## Java 17 Internals Java 17 introduces several new features, including sealed classes, switch expressions, and text blocks. These features improve the performance and readability of your code. Here’s an ASCII diagram of the Java 17 architecture:
+---------------+ | Java 17 | +---------------+ | Sealed | | Classes | +---------------+ | Switch | | Expressions| +---------------+ | Text Blocks| +---------------+
The following table compares the new features in Java 17 with their alternatives:
| Feature | Alternative |
|---|---|
| Sealed Classes | Abstract Classes |
| Switch Expressions | Switch Statements |
| Text Blocks | String Concatenation |
## Step-by-Step Implementation ### Step 1: Create a Sealed Class A sealed class is a class that can be extended only by a fixed set of classes. To create a sealed class, you need to use the sealed keyword.
public sealed class Animal permits Dog, Cat { public abstract void sound(); }
### Step 2: Implement a Switch Expression A switch expression is a more concise way to write a switch statement. To implement a switch expression, you need to use the switch keyword.
public String getAnimalSound(Animal animal) { return switch (animal) { case Dog d -> "Woof"; case Cat c -> "Meow"; }; }
### Step 3: Use a Text Block A text block is a way to represent a large block of text in a more readable way. To use a text block, you need to use the """ delimiter.
public String getHtml() { return """" <html> <body> <h1>Hello World</h1> </body> </html> """; }
The expected output of the above code is:
<html> <body> <h1>Hello World</h1> </body> </html>
## Complete Working Example The following is a complete working example that demonstrates the use of sealed classes, switch expressions, and text blocks:
package com.example.app; public sealed class Animal permits Dog, Cat { public abstract void sound(); } public final class Dog extends Animal { @Override public void sound() { System.out.println("Woof"); } } public final class Cat extends Animal { @Override public void sound() { System.out.println("Meow"); } } public class Main { public static void main(String[] args) { Animal dog = new Dog(); Animal cat = new Cat(); dog.sound(); cat.sound(); } }
You can test the above code using the following curl request:
curl -X GET "http://localhost:8080/animal/sound"
The expected response is:
Woof Meow
## Common Mistakes and How to Fix Them ### Mistake 1: Not Closing the Connection Pool If you don’t close the connection pool, you will see a java.sql.SQLException: Connection is closed error.
// WRONG - causes java.sql.SQLException: Connection is closed public Connection getConnection() { return DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "username", "password"); }
To fix this, you need to close the connection pool using the close() method.
public Connection getConnection() { Connection conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydb", "username", "password"); // ... conn.close(); return conn; }
### Mistake 2: Not Handling Exceptions If you don’t handle exceptions, you will see a java.lang.RuntimeException: Unhandled exception error.
// WRONG - causes java.lang.RuntimeException: Unhandled exception public void doSomething() { // ... throw new RuntimeException("Something went wrong"); }
To fix this, you need to handle exceptions using a try-catch block.
public void doSomething() { try { // ... } catch (RuntimeException e) { // Handle the exception } }
## Performance and Production Tips
Production tip: Use a connection pool to improve performance. Set the
hikari.maximum-pool-sizeproperty to 10 for a 4-core server.
Production tip: Use a load balancer to distribute traffic across multiple servers. This will improve performance and availability.
Production tip: Monitor your application’s performance using tools like Java Algorithms and Mastering SQL.
## Testing To test the above code, you can use the following JUnit 5 test:
public class AnimalTest { @Test public void testSound() { Animal dog = new Dog(); Animal cat = new Cat(); assertEquals("Woof", dog.sound()); assertEquals("Meow", cat.sound()); } }
You can also use Java Interview Questions to test your knowledge of Java. ## Key Takeaways * Use sealed classes to restrict inheritance. * Use switch expressions to simplify switch statements. * Use text blocks to represent large blocks of text. * Use a connection pool to improve performance. * Handle exceptions using a try-catch block. * Monitor your application’s performance using tools like More Java Tutorials and SOLID Design Principles in Java. For more information on Java, visit our Java Tutorials Hub.
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