How does PromptTemplate work in Spring AI?

PromptTemplate in Spring AI lets you define a prompt with named placeholders using {variableName} syntax and fill them in at runtime. This keeps prompt strings readable, testable as separate files, and decoupled from Java string concatenation.

// Inline template
PromptTemplate template = new PromptTemplate(
    "Explain {concept} to a {level} developer in plain English."
);
Prompt prompt = template.create(Map.of(
    "concept", "Java generics",
    "level", "junior"
));
String answer = chatModel.call(prompt)
    .getResult().getOutput().getContent();

For multi-line prompts you should externalise the template to a classpath resource:

// src/main/resources/prompts/code-review.st
PromptTemplate template = new PromptTemplate(
    new ClassPathResource("prompts/code-review.st")
);
Prompt prompt = template.create(Map.of("code", sourceCode));

When using ChatClient, the fluent API supports inline variable substitution without constructing a PromptTemplate object explicitly:

chatClient.prompt()
    .user(u -> u.text("Summarise {topic} in three bullet points")
               .param("topic", userInput))
    .call().content();

The {} placeholder syntax means you must escape any literal curly braces in your prompts as \{ and \}. Store template files in src/main/resources/prompts/ so prompt engineers can iterate on them without touching compiled Java.

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