Java String Templates: JDK 21 Preview and JDK 23 Removal
The compatibility point to check first
String Templates were a preview feature in JDK 21 and were previewed again in JDK 22. They were removed in JDK 23. Code using the STR processor from these previews cannot be enabled in JDK 23 simply by adding --enable-preview.
This article preserves the context of the original JDK 21 example and explains the constraints that matter when revisiting it. Preview syntax is tied to a particular JDK release and should not be assumed to become a permanent language feature.
Reproduce a JDK 21 preview example
Use a JDK 21 compiler and runtime for this historical example:
public class Example {
public static void main(String[] args) {
int x = 2;
int y = 3;
String result = STR."\{x} plus \{y} equals \{x + y}";
System.out.println(result);
}
}Compile and run with the matching release and preview flags:
javac --enable-preview --release 21 Example.java
java --enable-preview ExampleThe expected output is 2 plus 3 equals 5. Running a preview class on a different Java release is not a portable deployment strategy.
Standard formatting alternatives
For ordinary formatting, standard APIs remain useful. In the following examples, x is 2 and y is 3:
String result = "%1$d plus %2$d equals %3$d".formatted(x, y, x + y);String.formatted is available from Java 15. For older Java environments, use String.format with the same format string.
MessageFormat uses numbered placeholders and accepts the values as an object array:
java.text.MessageFormat format =
new java.text.MessageFormat("{0} plus {1} equals {2}");
String result = format.format(new Object[] {x, y, x + y});The array is significant: the original article’s format(x, y, x + y) call did not match that API. MessageFormat also has its own quoting and locale rules, so Java format strings and MessageFormat patterns are not interchangeable.
Formatting is not escaping
Interpolating a value into text does not automatically produce valid JSON or a safe SQL statement. A quote or a backslash inside a value can require encoding in the destination format.
Use a JSON serializer to produce JSON and parameter binding to construct SQL queries. Treat HTML output with the escaping rules of the rendering system. The convenience of a formatting expression does not replace those contracts.
Migration notes
- Identify whether existing code depends on a preview feature or a standard API.
- Record the compiler and runtime versions used by the example.
- Keep preview flags scoped to the matching build and test configuration.
- Replace preview syntax before moving to a release that no longer includes it.
- Keep shell or JShell output separate from compilable Java source.
The Korean article retains more of the original comparison examples; both editions include the removal and compatibility correction.
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