Modern Java

50. String Templates (Preview)

String templates (JEP 430/459) embed expressions directly in strings with \{expr} syntax using a template processor — safer than String.format because the processor validates and transforms the result before it becomes a string.

Java 21 preview (evolved through Java 22-25)

Explanation

String templates were introduced as a preview feature in Java 21 (JEP 430) and evolved through Java 22 (JEP 459). The key idea: a template processor (STR, FMT, or custom) receives both the literal string fragments and the interpolated values separately before forming the result — enabling sanitisation, escaping, or type-safe transformation.

STR."Hello \{name}!" produces a String. FMT."Pi = %.5f\{Math.PI}" applies printf-style formatting. Custom processors can prevent SQL injection or XSS by escaping values before interpolating.

Note: the exact final syntax may change as the feature finishes standardisation. The preview in Java 21/22 uses STR."..." processor syntax. Later preview revisions may alter this. Always check the JEP for the current version you are using.

Example

// Requires --enable-preview and the appropriate Java version
// java --enable-preview --source 21 StringTemplatesDemo.java

public class StringTemplatesDemo {
    public static void main(String[] args) {
        String name = "Alice";
        int    age  = 30;
        double pi   = Math.PI;

        // STR processor: simple string interpolation
        String greeting = STR."Hello, \{name}! You are \{age} years old.";
        System.out.println(greeting);
        // Hello, Alice! You are 30 years old.

        // Expressions in templates
        String calc = STR."Next year you will be \{age + 1}.";
        System.out.println(calc);

        // FMT processor: format specifiers
        String formatted = FMT."Pi to 4 places: %.4f\{pi}";
        System.out.println(formatted);
        // Pi to 4 places: 3.1416

        // Multi-line template (combined with text block syntax)
        String report = STR."""
                Name: \{name}
                Age:  \{age}
                """;
        System.out.print(report);
    }
}

Sources