Understanding Java String Templates (Preview Feature)

👋 Introduction

In Java, we often build strings using +, String.format(), or StringBuilder. While these work, they can be hard to read, prone to errors, and not very developer-friendly.

To fix this, Java 21 introduces a preview feature called String Templates – a new, modern way to format strings safely and clearly.

It’s cleaner. It’s smarter. It’s safer.

In this blog, we’ll break it down in simple terms for beginners.

Understanding Java String Templates (Preview Feature)

🔍 What Are String Templates?

String Templates are a new syntax for creating strings using placeholders. These placeholders are replaced with values at runtime — like modern templating in JavaScript or Python.

Java 21 introduces two important parts:

Template expressions
Processors

🧵 Why Do We Need String Templates?

Let’s compare:

🛠️ Traditional Way (Verbose & Hard to Maintain)

String name = "Ketan";

int age = 30;

String output = "Name: " + name + ", Age: " + age;

🚀 New Way Using String Templates

String name = "Ketan";

int age = 30;

String output = STR."Name: \{name}, Age: \{age}";

Just like that — it looks clean and readable and maintainable!

💡 How String Templates Work

Java uses template processors to interpret and replace placeholders (\{}).

In Java 21, the built-in processor is:

STR."..." // uses StringTemplate.STR

🧪 Example: Using String Templates

public class Main {

public static void main(String[] args) {

String user = "Alice";

int score = 95;

String message = STR."Hello \{user}, your score is \{score}.";

System.out.println(message);

}

✅ Output: Hello Alice, your score is 95.

🔐 Safety Benefits

String templates are:

✅ Type-safe (catch errors at compile-time)
✅ Null-safe
✅ Cleaner and more readable
✅ Secure (reduce injection risks in SQL, HTML)

📦 Advanced: Custom Processors

Besides STR, Java may allow custom processors for things like:

HTML templates
SQL queries
Logging formats

But for now, just focus on STR. for simple string output.

⚙️ Things to Remember

Feature	Supported?
Variables inside `{}`	✅ Yes
Multi-line strings	✅ Yes (with `"""`)
Template injection	❌ Prevented
Requires Java 21	✅ Yes (preview)

🔄 Template with Text Blocks

You can even use it with multiline strings:

String user = "Ketan";

String msg = STR."""

Dear \{user},

Welcome to Java 21!

""";

🧪 Enable in Java 21

Since this is a preview feature, you need to enable it while compiling:

javac --enable-preview --release 21 Main.java

java --enable-preview Main

🧾 Summary

Java 21 introduces String Templates as a cleaner way to format strings.
Use STR."..." with embedded placeholders like \{variable}.
It’s readable, safe, and beginner-friendly.
Works with simple values and even multiline text blocks.

💬 Final Thoughts

String Templates are a big step forward for modern Java.
If you’re learning Java 21, this is a great feature to adopt early. It makes your code cleaner, safer, and more expressive.

Reference : String Templates in Java 21