Add SpEL support for registered MethodHandles

This commit adds support for MethodHandles in SpEL, using the same
syntax as user-defined functions (which also covers reflective Methods).

The most benefit is expected with handles that capture a static method
with no arguments, or with fully bound handles (where all the arguments
have been bound, including a target instance as first bound argument
if necessary). Partially bound MethodHandle should also be supported.

A best effort approach is taken to detect varargs as there is no API
support to determine if an argument is a vararg or an explicit array,
unlike with Method. Argument conversions are also applied. Finally,
array repacking is not always necessary with varargs so it is only
performed when the vararg is the sole argument to the invoked method.

See gh-27099
Closes gh-30045

spring-expression/src/main/java/org/springframework/expression/spel/ast/FunctionReference.java

@@ -16,6 +16,8 @@
 
 package org.springframework.expression.spel.ast;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodType;
 import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.StringJoiner;
@@ -70,7 +72,17 @@ public class FunctionReference extends SpelNodeImpl {
 		if (value == TypedValue.NULL) {
 			throw new SpelEvaluationException(getStartPosition(), SpelMessage.FUNCTION_NOT_DEFINED, this.name);
 		}
-		if (!(value.getValue() instanceof Method function)) {
+		Object resolvedValue = value.getValue();
+		if (resolvedValue instanceof MethodHandle methodHandle) {
+			try {
+				return executeFunctionBoundMethodHandle(state, methodHandle);
+			}
+			catch (SpelEvaluationException ex) {
+				ex.setPosition(getStartPosition());
+				throw ex;
+			}
+		}
+		if (!(resolvedValue instanceof Method function)) {
 			// Possibly a static Java method registered as a function
 			throw new SpelEvaluationException(
 					SpelMessage.FUNCTION_REFERENCE_CANNOT_BE_INVOKED, this.name, value.getClass());
@@ -138,6 +150,78 @@ public class FunctionReference extends SpelNodeImpl {
 		}
 	}
 
+	/**
+	 * Execute a function represented as {@code java.lang.invoke.MethodHandle}.
+	 * Method types that take no arguments (fully bound handles or static methods
+	 * with no parameters) can use {@code #invoke()} which is the most efficient.
+	 * Otherwise, {@code #invokeWithArguments)} is used.
+	 * @param state the expression evaluation state
+	 * @param methodHandle the method to invoke
+	 * @return the return value of the invoked Java method
+	 * @throws EvaluationException if there is any problem invoking the method
+	 * @since 6.1.0
+	 */
+	private TypedValue executeFunctionBoundMethodHandle(ExpressionState state, MethodHandle methodHandle) throws EvaluationException {
+		Object[] functionArgs = getArguments(state);
+		MethodType declaredParams = methodHandle.type();
+		int spelParamCount = functionArgs.length;
+		int declaredParamCount = declaredParams.parameterCount();
+
+		boolean isSuspectedVarargs = declaredParams.lastParameterType().isArray();
+
+		if (spelParamCount < declaredParamCount || (spelParamCount > declaredParamCount
+				&& !isSuspectedVarargs)) {
+			//incorrect number, including more arguments and not a vararg
+			throw new SpelEvaluationException(SpelMessage.INCORRECT_NUMBER_OF_ARGUMENTS_TO_FUNCTION,
+					functionArgs.length, declaredParamCount);
+			//perhaps a subset of arguments was provided but the MethodHandle wasn't bound?
+		}
+
+		// simplest case: the MethodHandle is fully bound or represents a static method with no params:
+		if (declaredParamCount == 0) {
+			//note we consider MethodHandles not compilable
+			try {
+				return new TypedValue(methodHandle.invoke());
+			}
+			catch (Throwable ex) {
+				throw new SpelEvaluationException(getStartPosition(), ex, SpelMessage.EXCEPTION_DURING_FUNCTION_CALL,
+						this.name, ex.getMessage());
+			}
+			finally {
+				this.exitTypeDescriptor = null;
+				this.method = null;
+			}
+		}
+
+		// more complex case, we need to look at conversion and vararg repacking
+		Integer varArgPosition = null;
+		if (isSuspectedVarargs) {
+			varArgPosition = declaredParamCount - 1;
+		}
+		TypeConverter converter = state.getEvaluationContext().getTypeConverter();
+		boolean conversionOccurred = ReflectionHelper.convertAllMethodHandleArguments(converter,
+				functionArgs, methodHandle, varArgPosition);
+
+		if (isSuspectedVarargs && declaredParamCount == 1) {
+			//we only repack the varargs if it is the ONLY argument
+			functionArgs = ReflectionHelper.setupArgumentsForVarargsInvocation(
+					methodHandle.type().parameterArray(), functionArgs);
+		}
+
+		//note we consider MethodHandles not compilable
+		try {
+			return new TypedValue(methodHandle.invokeWithArguments(functionArgs));
+		}
+		catch (Throwable ex) {
+			throw new SpelEvaluationException(getStartPosition(), ex, SpelMessage.EXCEPTION_DURING_FUNCTION_CALL,
+					this.name, ex.getMessage());
+		}
+		finally {
+			this.exitTypeDescriptor = null;
+			this.method = null;
+		}
+	}
+
 	@Override
 	public String toStringAST() {
 		StringJoiner sj = new StringJoiner(",", "(", ")");

spring-expression/src/main/java/org/springframework/expression/spel/support/ReflectionHelper.java

@@ -16,6 +16,8 @@
 
 package org.springframework.expression.spel.support;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodType;
 import java.lang.reflect.Array;
 import java.lang.reflect.Executable;
 import java.lang.reflect.Method;
@@ -23,6 +25,7 @@ import java.util.List;
 import java.util.Optional;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.expression.EvaluationException;
 import org.springframework.expression.TypeConverter;
@@ -330,6 +333,91 @@ public abstract class ReflectionHelper {
 		return conversionOccurred;
 	}
 
+	/**
+	 * Takes an input set of argument values and converts them to the types specified as the
+	 * required parameter types. The arguments are converted 'in-place' in the input array.
+	 * @param converter the type converter to use for attempting conversions
+	 * @param arguments the actual arguments that need conversion
+	 * @param methodHandle the target MethodHandle
+	 * @param varargsPosition the known position of the varargs argument, if any
+	 * ({@code null} if not varargs)
+	 * @return {@code true} if some kind of conversion occurred on an argument
+	 * @throws EvaluationException if a problem occurs during conversion
+	 * @since 6.1.0
+	 */
+	public static boolean convertAllMethodHandleArguments(TypeConverter converter, Object[] arguments,
+			MethodHandle methodHandle, @Nullable Integer varargsPosition) throws EvaluationException {
+		boolean conversionOccurred = false;
+		final MethodType methodHandleArgumentTypes = methodHandle.type();
+		if (varargsPosition == null) {
+			for (int i = 0; i < arguments.length; i++) {
+				Class<?> argumentClass = methodHandleArgumentTypes.parameterType(i);
+				ResolvableType resolvableType = ResolvableType.forClass(argumentClass);
+				TypeDescriptor targetType = new TypeDescriptor(resolvableType, argumentClass, null);
+
+				Object argument = arguments[i];
+				arguments[i] = converter.convertValue(argument, TypeDescriptor.forObject(argument), targetType);
+				conversionOccurred |= (argument != arguments[i]);
+			}
+		}
+		else {
+			// Convert everything up to the varargs position
+			for (int i = 0; i < varargsPosition; i++) {
+				Class<?> argumentClass = methodHandleArgumentTypes.parameterType(i);
+				ResolvableType resolvableType = ResolvableType.forClass(argumentClass);
+				TypeDescriptor targetType = new TypeDescriptor(resolvableType, argumentClass, null);
+
+				Object argument = arguments[i];
+				arguments[i] = converter.convertValue(argument, TypeDescriptor.forObject(argument), targetType);
+				conversionOccurred |= (argument != arguments[i]);
+			}
+
+			final Class<?> varArgClass = methodHandleArgumentTypes.lastParameterType().getComponentType();
+			ResolvableType varArgResolvableType = ResolvableType.forClass(varArgClass);
+			TypeDescriptor varArgContentType = new TypeDescriptor(varArgResolvableType, varArgClass, null);
+
+			// If the target is varargs and there is just one more argument, then convert it here.
+			if (varargsPosition == arguments.length - 1) {
+				Object argument = arguments[varargsPosition];
+				TypeDescriptor sourceType = TypeDescriptor.forObject(argument);
+				if (argument == null) {
+					// Perform the equivalent of GenericConversionService.convertNullSource() for a single argument.
+					if (varArgContentType.getElementTypeDescriptor().getObjectType() == Optional.class) {
+						arguments[varargsPosition] = Optional.empty();
+						conversionOccurred = true;
+					}
+				}
+				// If the argument type is equal to the varargs element type, there is no need to
+				// convert it or wrap it in an array. For example, using StringToArrayConverter to
+				// convert a String containing a comma would result in the String being split and
+				// repackaged in an array when it should be used as-is.
+				else if (!sourceType.equals(varArgContentType.getElementTypeDescriptor())) {
+					arguments[varargsPosition] = converter.convertValue(argument, sourceType, varArgContentType);
+				}
+				// Possible outcomes of the above if-else block:
+				// 1) the input argument was null, and nothing was done.
+				// 2) the input argument was null; the varargs element type is Optional; and the argument was converted to Optional.empty().
+				// 3) the input argument was correct type but not wrapped in an array, and nothing was done.
+				// 4) the input argument was already compatible (i.e., array of valid type), and nothing was done.
+				// 5) the input argument was the wrong type and got converted and wrapped in an array.
+				if (argument != arguments[varargsPosition] &&
+						!isFirstEntryInArray(argument, arguments[varargsPosition])) {
+					conversionOccurred = true; // case 5
+				}
+			}
+			// Otherwise, convert remaining arguments to the varargs element type.
+			else {
+				Assert.state(varArgContentType != null, "No element type");
+				for (int i = varargsPosition; i < arguments.length; i++) {
+					Object argument = arguments[i];
+					arguments[i] = converter.convertValue(argument, TypeDescriptor.forObject(argument), varArgContentType);
+					conversionOccurred |= (argument != arguments[i]);
+				}
+			}
+		}
+		return conversionOccurred;
+	}
+
 	/**
 	 * Check if the supplied value is the first entry in the array represented by the possibleArray value.
 	 * @param value the value to check for in the array

spring-expression/src/main/java/org/springframework/expression/spel/support/StandardEvaluationContext.java

@@ -16,6 +16,7 @@
 
 package org.springframework.expression.spel.support;
 
+import java.lang.invoke.MethodHandle;
 import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.List;
@@ -251,6 +252,10 @@ public class StandardEvaluationContext implements EvaluationContext {
 		this.variables.put(name, method);
 	}
 
+	public void registerFunction(String name, MethodHandle methodHandle) {
+		this.variables.put(name, methodHandle);
+	}
+
 	@Override
 	@Nullable
 	public Object lookupVariable(String name) {

spring-expression/src/test/java/org/springframework/expression/spel/ExpressionLanguageScenarioTests.java

@@ -188,6 +188,45 @@ public class ExpressionLanguageScenarioTests extends AbstractExpressionTests {
 		}
 	}
 
+	/**
+	 * Scenario: looking up your own MethodHandles and calling them from the expression
+	 */
+	@Test
+	public void testScenario_RegisteringJavaMethodsAsMethodHandlesAndCallingThem() throws SecurityException, NoSuchMethodException {
+		try {
+			// Create a parser
+			SpelExpressionParser parser = new SpelExpressionParser();
+			//this.context is already populated with all relevant MethodHandle examples
+
+			Expression expr = parser.parseRaw("#message('Message with %s words: <%s>', 2, 'Hello World', 'ignored')");
+			Object value = expr.getValue(this.context);
+			assertThat(value).isEqualTo("Message with 2 words: <Hello World>");
+
+			expr = parser.parseRaw("#messageTemplate('bound', 2, 'Hello World', 'ignored')");
+			value = expr.getValue(this.context);
+			assertThat(value).isEqualTo("This is a bound message with 2 words: <Hello World>");
+
+			expr = parser.parseRaw("#messageBound()");
+			value = expr.getValue(this.context);
+			assertThat(value).isEqualTo("This is a prerecorded message with 3 words: <Oh Hello World>");
+
+			Expression staticExpr = parser.parseRaw("#messageStatic('Message with %s words: <%s>', 2, 'Hello World', 'ignored')");
+			Object staticValue = staticExpr.getValue(this.context);
+			assertThat(staticValue).isEqualTo("Message with 2 words: <Hello World>");
+
+			staticExpr = parser.parseRaw("#messageStaticTemplate('bound', 2, 'Hello World', 'ignored')");
+			staticValue = staticExpr.getValue(this.context);
+			assertThat(staticValue).isEqualTo("This is a bound message with 2 words: <Hello World>");
+
+			staticExpr = parser.parseRaw("#messageStaticBound()");
+			staticValue = staticExpr.getValue(this.context);
+			assertThat(staticValue).isEqualTo("This is a prerecorded message with 3 words: <Oh Hello World>");
+		}
+		catch (EvaluationException | ParseException ex) {
+			throw new AssertionError(ex.getMessage(), ex);
+		}
+	}
+
 	/**
 	 * Scenario: add a property resolver that will get called in the resolver chain, this one only supports reading.
 	 */

spring-expression/src/test/java/org/springframework/expression/spel/SpelDocumentationTests.java

@@ -16,6 +16,9 @@
 
 package org.springframework.expression.spel;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Date;
@@ -415,6 +418,38 @@ class SpelDocumentationTests extends AbstractExpressionTests {
 		assertThat(helloWorldReversed).isEqualTo("dlrow olleh");
 	}
 
+	@Test
+	void methodHandlesNotBound() throws Throwable {
+		ExpressionParser parser = new SpelExpressionParser();
+		StandardEvaluationContext context = new StandardEvaluationContext();
+
+		MethodHandle mh = MethodHandles.lookup().findVirtual(String.class, "formatted",
+				MethodType.methodType(String.class, Object[].class));
+		context.setVariable("message", mh);
+
+		String message = parser.parseExpression("#message('Simple message: <%s>', 'Hello World', 'ignored')")
+				.getValue(context, String.class);
+		assertThat(message).isEqualTo("Simple message: <Hello World>");
+	}
+
+	@Test
+	void methodHandlesFullyBound() throws Throwable {
+		ExpressionParser parser = new SpelExpressionParser();
+		StandardEvaluationContext context = new StandardEvaluationContext();
+
+		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);
+
+		String message = parser.parseExpression("#message()")
+				.getValue(context, String.class);
+		assertThat(message).isEqualTo("This is a prerecorded message with 3 words: <Oh Hello World!>");
+	}
+
 	// 7.5.10
 
 	@Test

spring-expression/src/test/java/org/springframework/expression/spel/TestScenarioCreator.java

@@ -16,6 +16,9 @@
 
 package org.springframework.expression.spel;
 
+import java.lang.invoke.MethodHandle;
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.MethodType;
 import java.util.Arrays;
 import java.util.GregorianCalendar;
 
@@ -37,6 +40,12 @@ class TestScenarioCreator {
 		setupRootContextObject(testContext);
 		populateVariables(testContext);
 		populateFunctions(testContext);
+		try {
+			populateMethodHandles(testContext);
+		}
+		catch (NoSuchMethodException | IllegalAccessException e) {
+			throw new RuntimeException(e);
+		}
 		return testContext;
 	}
 
@@ -62,6 +71,36 @@ class TestScenarioCreator {
 		}
 	}
 
+	/**
+	 * Register some Java {@code MethodHandle} as well known functions that can be called from an expression.
+	 * @param testContext the test evaluation context
+	 */
+	private static void populateMethodHandles(StandardEvaluationContext testContext) throws NoSuchMethodException, IllegalAccessException {
+		// #message(template, args...)
+		MethodHandle message = MethodHandles.lookup().findVirtual(String.class, "formatted",
+				MethodType.methodType(String.class, Object[].class));
+		testContext.registerFunction("message", message);
+		// #messageTemplate(args...)
+		MethodHandle messageWithParameters = message.bindTo("This is a %s message with %s words: <%s>");
+		testContext.registerFunction("messageTemplate", messageWithParameters);
+		// #messageTemplateBound()
+		MethodHandle messageBound = messageWithParameters
+				.bindTo(new Object[] { "prerecorded", 3, "Oh Hello World", "ignored"});
+		testContext.registerFunction("messageBound", messageBound);
+
+		//#messageStatic(template, args...)
+		MethodHandle messageStatic = MethodHandles.lookup().findStatic(TestScenarioCreator.class,
+				"message", MethodType.methodType(String.class, String.class, String[].class));
+		testContext.registerFunction("messageStatic", messageStatic);
+		//#messageStaticTemplate(args...)
+		MethodHandle messageStaticPartiallyBound = messageStatic.bindTo("This is a %s message with %s words: <%s>");
+		testContext.registerFunction("messageStaticTemplate", messageStaticPartiallyBound);
+		//#messageStaticBound()
+		MethodHandle messageStaticFullyBound = messageStaticPartiallyBound
+				.bindTo(new String[] { "prerecorded", "3", "Oh Hello World", "ignored"});
+		testContext.registerFunction("messageStaticBound", messageStaticFullyBound);
+	}
+
 	/**
 	 * Register some variables that can be referenced from the tests
 	 * @param testContext the test evaluation context
@@ -117,4 +156,8 @@ class TestScenarioCreator {
 		return String.valueOf(i) + "-" + Arrays.toString(strings);
 	}
 
+	public static String message(String template, String... args) {
+		return template.formatted((Object[]) args);
+	}
+
 }