String Templates

4 min readJun 8, 2026

1. What are String Templates?

String Templates are a preview feature (Java 21, refined in Java 22) that allow embedding expressions into strings safely and declaratively.

Think of it as:

String interpolation + validation + processing

2. Problem Before String Templates

Before Java 21:

String concatenation

String name = "Nazeer";
String s = "Hello " + name;

String.format (unsafe, error-prone)

String s = String.format("Hello %s", name);

Problems:

No compile-time validation
SQL injection risks
Hard to read for complex strings

3. Basic Syntax (Core Concept)

String name = "Nazeer";
String message = STR."Hello {name}";

STR → template processor
\{} → embedded expression

4. Key Components

4.1 Template Expression

STR."Hello {name}"

Breakdown:

STR → processor
"Hello \{name}" → template
\{name} → expression

4.2 Template Processor

Acts like an interpreter for templates

String result = STR."Value = {10 + 20}";

Output:

Value = 30

5. Built-in Processors

5.1 STR (Default)

String s = STR."Sum = {10 + 20}";

Equivalent to:

“Sum = “ + (10 + 20);

5.2 RAW (Advanced)

StringTemplate template = RAW."Hello {name}";

Gives:

fragments
values

Used for custom processing

6. Multiple Expressions

String name = "Nazeer";
int age = 30;
String s = STR."{name} is {age} years old";

Clean & readable

7. Expressions Power (Very Important)

String result = STR."Square = {Math.pow(5,2)}";
String result = STR."Upper = {name.toUpperCase()}";

Any Java expression allowed

8. Multi-line Templates

String sql = STR."""
SELECT *
FROM users
WHERE name = {name}
""";

Combines:

text blocks
interpolation

9. The Real Power: Custom Processors

This is where String Templates shine (interview gold)

Example: SQL Injection Safe Processor

StringTemplate.Processor<String, RuntimeException> SQL =
template -> {
  StringBuilder sb = new StringBuilder();
  Iterator<String> fragments = template.fragments().iterator();
  Iterator<Object> values = template.values().iterator();

  while (fragments.hasNext()) {
    sb.append(fragments.next());
    if (values.hasNext()) {
      sb.append("?"); // replace with placeholder
    }
  }

  return sb.toString();
};

String query = SQL."SELECT * FROM users WHERE name = {name}";

Output:

SELECT * FROM users WHERE name = ?

Prevents SQL injection

Example: Logging Template

var LOG = StringTemplate.Processor.of(
template -> template.interpolate()
);
String msg = LOG."User {name} logged in at {System.currentTimeMillis()}";

10. StringTemplate API (Important)

Core Interface

public interface StringTemplate {
List<String> fragments();
List<Object> values();
}

Example

StringTemplate st = RAW."Hello {name}";

Gives:

fragments: [“Hello “, “”]

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values: [“Nazeer”]

11. Internal Working (Senior-Level Insight)

Template:

"Hello \{name}"

Becomes:

fragments → [“Hello “, “”]
values → [name]

Processor:

combine(fragments, values)

12. Security Advantage

Without templates (danger)

String query = "SELECT * FROM users WHERE name = '" + name + "'";

SQL Injection risk

With templates + processor

SQL."SELECT * FROM users WHERE name = {name}";

Safe parameter binding

13. Performance Consideration

Compiled efficiently → no major overhead

14. Limitations (Important)

Preview Feature

Java 21 → Preview
Java 22 → Preview (refined)
May change in future

Must enable preview

— enable-preview — release 21

Not widely adopted yet

Limited framework support

15. Real-World Use Cases

1. SQL Queries

SQL."SELECT * FROM orders WHERE id = {id}";

2. JSON Building

String json = STR."""
{
"name": "{name}",
"age": {age}
}
""";

3. Logging

logger.info(STR."User {userId} logged in");

4. REST Responses

return STR."Hello {user.getName()}";

16. Comparison