Functions

You can extend SpEL by registering user-defined functions that can be called within expressions by using the #functionName(…) syntax. Functions can be registered as variables in EvaluationContext implementations via the setVariable() method.

StandardEvaluationContext also defines registerFunction(…) methods that provide a convenient way to register a function as a java.lang.reflect.Method or a java.lang.invoke.MethodHandle.

Since functions share a common namespace with variables in the evaluation context, care must be taken to ensure that function names and variable names do not overlap.

The following example shows how to register a user-defined function to be invoked via reflection using a java.lang.reflect.Method:

Java
Kotlin

Method method = ...;

EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
context.setVariable("myFunction", method);

val method: Method = ...

val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()
context.setVariable("myFunction", method)

For example, consider the following utility method that reverses a string:

Java
Kotlin

public abstract class StringUtils {

	public static String reverseString(String input) {
		return new StringBuilder(input).reverse().toString();
	}
}

fun reverseString(input: String): String {
	return StringBuilder(input).reverse().toString()
}

You can register and use the preceding method, as the following example shows:

Java
Kotlin

ExpressionParser parser = new SpelExpressionParser();

EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();
context.setVariable("reverseString",
		StringUtils.class.getMethod("reverseString", String.class));

// evaluates to "olleh"
String helloWorldReversed = parser.parseExpression(
		"#reverseString('hello')").getValue(context, String.class);

val parser = SpelExpressionParser()

val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()
context.setVariable("reverseString", ::reverseString.javaMethod)

// evaluates to "olleh"
val helloWorldReversed = parser.parseExpression(
		"#reverseString('hello')").getValue(context, String::class.java)

A function can also be registered as a java.lang.invoke.MethodHandle. This enables potentially more efficient use cases if the MethodHandle target and parameters have been fully bound prior to registration; however, partially bound handles are also supported.

Consider the String#formatted(String, Object…) instance method, which produces a message according to a template and a variable number of arguments.

You can register and use the formatted method as a MethodHandle, as the following example shows:

Java
Kotlin

ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

MethodHandle mh = MethodHandles.lookup().findVirtual(String.class, "formatted",
		MethodType.methodType(String.class, Object[].class));
context.setVariable("message", mh);

// evaluates to "Simple message: <Hello World>"
String message = parser.parseExpression("#message('Simple message: <%s>', 'Hello World', 'ignored')")
		.getValue(context, String.class);

val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

val mh = MethodHandles.lookup().findVirtual(String::class.java, "formatted",
		MethodType.methodType(String::class.java, Array<Any>::class.java))
context.setVariable("message", mh)

// evaluates to "Simple message: <Hello World>"
val message = parser.parseExpression("#message('Simple message: <%s>', 'Hello World', 'ignored')")
		.getValue(context, String::class.java)

As hinted above, binding a MethodHandle and registering the bound MethodHandle is also supported. This is likely to be more performant if both the target and all the arguments are bound. In that case no arguments are necessary in the SpEL expression, as the following example shows:

Java
Kotlin

ExpressionParser parser = new SpelExpressionParser();
EvaluationContext context = SimpleEvaluationContext.forReadOnlyDataBinding().build();

String template = "This is a %s message with %s words: <%s>";
Object varargs = new Object[] { "prerecorded", 3, "Oh Hello World!", "ignored" };
MethodHandle mh = MethodHandles.lookup().findVirtual(String.class, "formatted",
		MethodType.methodType(String.class, Object[].class))
		.bindTo(template)
		.bindTo(varargs); //here we have to provide arguments in a single array binding
context.setVariable("message", mh);

// evaluates to "This is a prerecorded message with 3 words: <Oh Hello World!>"
String message = parser.parseExpression("#message()")
		.getValue(context, String.class);

val parser = SpelExpressionParser()
val context = SimpleEvaluationContext.forReadOnlyDataBinding().build()

val template = "This is a %s message with %s words: <%s>"
val varargs = arrayOf("prerecorded", 3, "Oh Hello World!", "ignored")

val mh = MethodHandles.lookup().findVirtual(String::class.java, "formatted",
		MethodType.methodType(String::class.java, Array<Any>::class.java))
		.bindTo(template)
		.bindTo(varargs) //here we have to provide arguments in a single array binding
context.setVariable("message", mh)

// evaluates to "This is a prerecorded message with 3 words: <Oh Hello World!>"
val message = parser.parseExpression("#message()")
		.getValue(context, String::class.java)