Avoid NPE when registering a SpEL MethodFilter

Attempting to register a custom MethodFilter with a
StandardEvaluationContext after invoking setMethodResolvers() with a
custom list of MethodResolver instances results in a
NullPointerException. Based on the current documentation in
StandardEvaluationContext it is unclear what the expected behavior
should be, but either the implementation is broken, or the use case is
unsupported. In either case, allowing a NullPointerException to be
thrown is inappropriate.

This commit documents the fact that the SpEL MethodFilter is intended to
be used with the ReflectiveMethodResolver. Furthermore,
StandardEvaluationContext.registerMethodFilter() now throws an
IllegalStateException if the user attempts to set a filter after having
registered a custom set of resolvers.

Issue: SPR-9621

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

@@ -18,15 +18,22 @@ package org.springframework.expression.spel;
 
 import static org.junit.Assert.*;
 
+import java.lang.reflect.Method;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
 
 import org.junit.Test;
 
+import org.springframework.core.convert.TypeDescriptor;
+import org.springframework.expression.AccessException;
 import org.springframework.expression.EvaluationContext;
 import org.springframework.expression.EvaluationException;
 import org.springframework.expression.Expression;
 import org.springframework.expression.ExpressionParser;
+import org.springframework.expression.MethodExecutor;
+import org.springframework.expression.MethodFilter;
+import org.springframework.expression.MethodResolver;
 import org.springframework.expression.ParseException;
 import org.springframework.expression.spel.standard.SpelExpression;
 import org.springframework.expression.spel.standard.SpelExpressionParser;
@@ -98,8 +105,8 @@ public class EvaluationTests extends ExpressionTestCase {
 	}
 
 
+	@SuppressWarnings("rawtypes")
 	static class TestClass {
-
 		public Foo wibble;
 		private Foo wibble2;
 		public Map map;
@@ -571,4 +578,48 @@ public class EvaluationTests extends ExpressionTestCase {
 		assertNull(exp.getValue());
 	}
 
+	/**
+	 * Verifies behavior requested in SPR-9621.
+	 */
+	@Test
+	public void customMethodFilter() throws Exception {
+		StandardEvaluationContext context = new StandardEvaluationContext();
+
+		// Register a custom MethodResolver...
+		List<MethodResolver> customResolvers = new ArrayList<MethodResolver>();
+		customResolvers.add(new CustomMethodResolver());
+		context.setMethodResolvers(customResolvers);
+
+		// or simply...
+		// context.setMethodResolvers(new ArrayList<MethodResolver>());
+
+		// Register a custom MethodFilter...
+		MethodFilter filter = new CustomMethodFilter();
+		try {
+			context.registerMethodFilter(String.class, filter);
+			fail("should have failed");
+		} catch (IllegalStateException ise) {
+			assertEquals(
+					"Method filter cannot be set as the reflective method resolver is not in use",
+					ise.getMessage());
+		}
+	}
+
+	static class CustomMethodResolver implements MethodResolver {
+
+		public MethodExecutor resolve(EvaluationContext context,
+				Object targetObject, String name,
+				List<TypeDescriptor> argumentTypes) throws AccessException {
+			return null;
+		}
+	}
+
+	static class CustomMethodFilter implements MethodFilter {
+
+		public List<Method> filter(List<Method> methods) {
+			return null;
+		}
+
+	}
+
 }