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String Templates (JEP 430)

String templates, previewed in Java 21, are a mechanism for producing objects from templates that contain string fragments and embedded expressions. The syntax is different from that of other languages, but the differences are minor and make sense for Java. The JDK provides processors for plain interpolation and formatted output. It is easy to implement your own processors.

String Interpolation

Many strings are composed of fixed and variable parts. In Java, such strings are traditionally formed through concatenation:

String message = "Hello, " + name + "! Next year, you'll be " + (age + 1) + ".";

That is tedious to read and write, and a bit error-prone since it is easy to mess up spaces and quotation marks.

Formatted output can be a better alternative:

message = "Hello, %s! Next year, you'll be %d.".formatted(name, age + 1);

MessageFormat is similar, but optimized for internationalization:

message = MessageFormat.format("Hello, {0}! Next year, you''ll be {1,choice,0≤{1}|50<even wiser}.", 
   name, age + 1);

In both cases, the contents of the composite string is easier to make out, but the variable parts are separated from their final location.

Many programming languages provide interpolation of expressions that are embedded inside a string. (The Merriam-Webster dictionary defines “interpolate” as “to insert between other things or parts”.) Java 21 adds a preview feature with the same capability. Here is how it looks like:

message = STR."Hello, \{name}! Next year, you'll be \{age + 1}.";

STR is a template processor and "Hello, \{name}! Next year, you'll be \{age + 1}." is a string template with embedded expressions \{name} and \{age + 1}. The entire expression on the right hand side of the assignment is a template expression.

Here is a sandbox to play with these alternatives.

import java.text.MessageFormat; public class Demo { public static void main(String[] args) { String name = "Fred"; int age = 42; String message = "Hello, " + name + "! Next year, you'll be " + (age + 1) + "."; System.out.println(message); message = "Hello, %s! Next year, you'll be %d.".formatted(name, age + 1); System.out.println(message); message = MessageFormat.format("Hello, {0}! Next year, you''ll be {1,choice,0≤{1}|50<even wiser}.", name, age + 1); System.out.println(message); message = STR."Hello, \{name}! Next year, you'll be \{age + 1}."; System.out.println(message); } }

Template Expressions

A template expression starts with an expression whose value is a template processor, an instance of a class that implements the StringTemplate.Processor interface. That interface has a single method process.

The template processor is followed by a dot, which the JEP describes unhelpfully as “a dot character (U+002E), as seen in other kinds of expressions.” Why a dot? Perhaps to remind the reader that this is a shorthand for a method call:

STR.process(templateArgument)

To the right of the dot is the template argument, which can be one of four things:

  1. A string template (as in the example)
  2. A text block template
  3. A string literal
  4. A text block

A string template is delimited by single quotes, just like a string literal, but it contains at least one embedded expression. (As it happens, \{ is not a valid escape sequence in a string literal.)

Similarly, a text block template is just like a text block, but with embedded expressions. Here is an example:

STR."""
<div>
   <p class="greeting">Hello, \{name}!</p>
   <p>Next year, you'll be \{age + 1}.</p>
</div>"""

In both string templates and text block templates, you can add comments and line breaks inside embedded expressions:

String message = STR."Hello, \{ name /* Ok to add a comment */ }! Next year, you'll be \{
      age + 1 // Ok to add line breaks
   }.";

A template is not a String. It is an instance of the StringTemplate class. You will see soon what that class does.

Unlike a string literal or text block, a template is only a valid expression when it follows a template processor and a dot. The processor is responsible for turning the text and embedded expressions of the template into a result. Without a processor, a template doesn’t have a value.

You are also allowed to place a string literal or text block after a template processor and a dot. You can regard that as the special case of a template with zero embedded expressions. It can also be a temporary starting point that is later turned into a template:

message = STR."Hello, TODO! Next year, you'll be TODO.";

This sandbox shows the four kinds of template arguments.

public class TemplateArguments { public static void main(String[] args) { String name = "Fred"; int age = 42; // 1. String template String message = STR."Hello, \{ name /* Ok to add a comment */ }! Next year, you'll be \{ age + 1 // Ok to add line breaks }."; System.out.println(message); // 2. Text block template String div = STR.""" <div> <p class="greeting">Hello, \{name}!</p> <p>Next year, you'll be \{age + 1}.</p> </div>"""; System.out.println(div); // 3. String literal message = STR."Hello, TODO! Next year, you'll be TODO."; System.out.println(message); // 4. Text block div = STR.""" <div> <p class="greeting">Hello, TODO!</p> <p>Next year, you'll be TODO.</p> </div>"""; System.out.println(div); } }

Predefined Template Processors

The Java 21 API contains three template processors: the interpolating STR processor that you have already seen, as well as processors for formatting and for obtaining an unprocessed template.

The STR processor, a static field of the class java.lang.StringTemplate is special—it is always imported. In other words, every Java file has these automatic imports:

import java.lang.*;
import static java.lang.StringTemplate.STR;

The other processors must be imported manually:

import static java.util.FormatProcessor.FMT;
import static java.lang.StringTemplate.RAW;

The FMT processor carries out printf style formatting:

String line = FMT."%-20s\{item.description} | %5d\{item.quantity} | %10.2f\{item.price}%n";

The format specifiers must immediately precede the embedded expressions.

The RAW processor yields the template argument as a StringTemplate instance:

StringTemplate template = RAW."Hello, \{name}! Next year, you'll be \{age + 1}.";

See the following section for more information about the StringTemplate class.

Note that a template processor can produce an object of any class. The FMT processor yields a String, but the RAW processor yields an object of the class StringTemplate.

Here is a sandbox with these examples. Try adding a space before or after a format specifier. Also try reassigning item after the raw template was formed. Are the values updated? Should they be?

import static java.util.FormatProcessor.FMT; import static java.lang.StringTemplate.RAW; public class TemplateProcessors { record Item(String description, int quantity, double price) {} public static void main(String[] args) { var item = new Item("Blackwell Toaster", 2, 29.95); String line = FMT."%-20s\{item.description()} | %5d\{item.quantity()} | %10.2f\{item.price()}%n"; System.out.print(line); item = new Item("Zappa Microwave Oven", 1, 109.95); // TODO What happens if you add a space before or after %-20s? line = FMT."%-20s\{item.description()} | %5d\{item.quantity()} | %10.2f\{item.price()}%n"; System.out.print(line); StringTemplate template = RAW."%-20s\{item.description()} | %5d\{item.quantity()} | %10.2f\{item.price()}%n"; // TODO What happens if you assign another instance to item here? System.out.println(template); } }

The StringTemplate class

When a template expression is evaluated, the template arguments are placed in an object that is then passed to the format method of the template processor.

The name StringTemplate is perhaps unfortunate. Any of the four possible template argument types is turned into a StringTemplate instance. TemplateArguments might have been a better choice for the class name.

A StringTemplate instance stores this information:

The fragments and values methods yield lists of fragments and values.

String name = "Fred";
int age = 42;
StringTemplate template = RAW."Hello, \{name}! Next year, you'll be \{age + 1}.";
template.fragments() // ["Hello, ", "! Next year, you'll be ", "."]
template.values() // ["Fred", 43]

Note that there is one more fragment than there are values.

The static interpolate method merges a list of fragments and values. As you will see in the next section, this is useful for implementing your own template processors.

I don’t think that many programmers will create StringTemplate instances, but there are three ways to do so:

There are two combine methods, with parameter types StringTemplate... and List<StringTemplate>.

Finally, the process method lets you apply a processor, just in case you prefer

template.process(processor)

over

processor.process(template)

This sandbox shows the API in action, even though you probably only need to worry about the first two methods.

import static java.lang.StringTemplate.RAW; import java.util.List; public class StringTemplateDemo { public static void main(String[] args) { String name = "Fred"; int age = 42; StringTemplate template = RAW."Hello, \{name}! Next year, you'll be \{age + 1}."; System.out.println(template.fragments()); System.out.println(template.values()); // Two interpolate methods: one static, one instance System.out.println(StringTemplate.interpolate( template.fragments().stream().map(String::toUpperCase).toList(), template.values())); System.out.println(StringTemplate.of( template.fragments().stream().map(String::toUpperCase).toList(), template.values()).interpolate()); // Demo of the of and combine methods. // For each entry in System.getProperties(), one StringTemplate is created. // They are all combined and then processed with the STR processor // Note that each StringTemplate has two fragments, and combine fuses // the second fragment with the first fragment of its successor. template = StringTemplate.combine( System.getProperties() .entrySet() .stream() .map(e -> StringTemplate.of(List.of(e.getKey() + ": ", "\n"), List.of(e.getValue()))) .toList()); System.out.println(template.process(STR)); } }

Writing Your Own Template Processor

The format method of a template processor turns StringTemplate instances into objects. Since StringTemplate.Processor is a functional interface, you can construct an instance from a lambda expression. Here is a simple example that places the values in boxes:

public class MyFirstTemplateProcessor { public static StringTemplate.Processor<String, RuntimeException> BOX = (StringTemplate template) -> { var result = new StringBuilder(); for (int i = 0; i < template.fragments().size(); i++) { if (i > 0) { result.append('['); result.append("" + template.values().get(i - 1)); result.append(']'); } result.append(template.fragments().get(i)); } return result.toString(); }; public static void main(String[] args) { String name = "Fred"; int age = 42; System.out.println(BOX."Hello, \{name}! Next year, you'll be \{age + 1}."); } }

StringTemplate.Processor is a generic class with two type parameters: the result type and the type of the exception that the processor can throw.

Here, the values are transformed and then interpolated with the fragments. The interpolate method is helpful in this common situation:

public static StringTemplate.Processor<String, RuntimeException> BOX =
   template -> StringTemplate.interpolate(template.fragments(),
      template.values().stream().map(v -> "[" + v + "]").toList());

As an aside, in the JEP, all template processors are written in uppercase, even if they are not static variables. This is not a requirement. The processor is an expression like any other. If a processor is a local variable, or a method invocation, it is perfectly fine to use lowercase letters. For example:

public static StringTemplate.Processor<String, RuntimeException> box(String left, String right) {
   return template -> StringTemplate.interpolate(template.fragments(),
      template.values().stream().map(v -> left + v + right).toList());
}
...
String result = box("{", "}")."Hello, \{name}! Next year, you'll be \{age + 1}.";

String processors can protect against injection attacks. When creating HTML, XML, JSON, SQL, and so on, from arbitrary values, you want to escape special characters such as quotation marks and bracket delimiters. The JEP has a useful example of using a PreparedStatement for a SQL query. This sandbox shows a simpler example for processing XML.

import java.io.*; import javax.xml.parsers.*; import org.w3c.dom.*; import org.w3c.dom.ls.*; import org.xml.sax.*; public class Xml { private static String escape(String str) { return str.replace("&", "&amp;") .replace("<", "&lt;") .replace(">", "&gt;") .replace("\"", "&quot;") .replace("'", "&apos;"); } public static StringTemplate.Processor<Document, Exception> XML = template -> { String escaped = StringTemplate.interpolate(template.fragments(), template.values().stream().map(String::valueOf).map(Xml::escape).toList()); return parse(escaped); }; // Here are two methods for parsing and printing XML. Don't pay too much attention // to the byzantine API from an earlier age... private static Document parse(String xml) throws SAXException, IOException, ParserConfigurationException { DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = factory.newDocumentBuilder(); return builder.parse(new ByteArrayInputStream(xml.getBytes())); } private static String prettyPrint(Document doc) { DOMImplementation impl = doc.getImplementation(); var implLS = (DOMImplementationLS) impl.getFeature("LS", "3.0"); LSSerializer ser = implLS.createLSSerializer(); ser.getDomConfig().setParameter("format-pretty-print", true); return ser.writeToString(doc); } public static void main(String[] args) throws Exception { String name = "J.R. \"Bob\" Dobbs <dobbs@subgenius.com>"; int age = 42; Document doc = XML.""" <div> <p class="greeting">Hello, \{name}!</p> <p>Next year, you'll be \{age + 1}.</p> </div>"""; System.out.println(prettyPrint(doc)); } }

Now it’s your turn. Complete the code below so that one can write a Map<String, Object> as

MAP."key1 \{value1} key2 \{value2} ..."

Trim the fragments and ignore the last one.

import java.time.*; import java.util.*; public class Challenge { public static ... MAP = ...; public static void main(String[] args) { LocalDate today = LocalDate.now(); Map<String, Object> todayMap = MAP.""" day \{today.getDayOfMonth()} month \{today.getMonth()} year \{today.getYear()} """; System.out.println(todayMap); } }
Show Solution
public static StringTemplate.Processor<Map<String, Object>, RuntimeException> MAP = template -> {
   List<Object> values = template.values();
   var result = new HashMap<String, Object>();
   for (int i = 0; i < values.size(); i++)
      result.put(template.fragments().get(i).trim(), values.get(i));
   return result;
};

Conclusion

String templates provide interpolation in a way that should be familiar to anyone who has used this feature in another programming language. Here are the highlights:

References