Reject null return value from MethodReplacer for primitive return type

This commit throws an exception instead of silently converting a null
return value from a MethodReplacer to a primitive 0/false value.

See gh-32412

spring-beans/src/main/java/org/springframework/beans/factory/support/CglibSubclassingInstantiationStrategy.java

@@ -18,6 +18,7 @@ package org.springframework.beans.factory.support;
 
 import java.lang.reflect.Constructor;
 import java.lang.reflect.Method;
+import java.util.Objects;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -275,13 +276,24 @@ public class CglibSubclassingInstantiationStrategy extends SimpleInstantiationSt
 			this.owner = owner;
 		}
 
+		@Nullable
 		@Override
 		public Object intercept(Object obj, Method method, Object[] args, MethodProxy mp) throws Throwable {
 			ReplaceOverride ro = (ReplaceOverride) getBeanDefinition().getMethodOverrides().getOverride(method);
 			Assert.state(ro != null, "ReplaceOverride not found");
 			// TODO could cache if a singleton for minor performance optimization
 			MethodReplacer mr = this.owner.getBean(ro.getMethodReplacerBeanName(), MethodReplacer.class);
-			return mr.reimplement(obj, method, args);
+			return processReturnType(method, mr.reimplement(obj, method, args));
+		}
+
+		@Nullable
+		private <T> T processReturnType(Method method, @Nullable T returnValue) {
+			Class<?> returnType = method.getReturnType();
+			if (returnType != void.class && returnType.isPrimitive()) {
+				return Objects.requireNonNull(returnValue, () -> "Null return value from replacer does not match primitive return type for: " + method);
+			}
+
+			return returnValue;
 		}
 	}
 

spring-beans/src/test/java/org/springframework/beans/factory/support/CglibSubclassingInstantiationStrategyTests.java

@@ -0,0 +1,120 @@
+package org.springframework.beans.factory.support;
+
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
+
+import java.lang.reflect.Method;
+import java.util.Map;
+import java.util.stream.Stream;
+
+import org.assertj.core.api.ThrowableAssert;
+import org.junit.jupiter.api.Test;
+import org.springframework.lang.Nullable;
+
+class CglibSubclassingInstantiationStrategyTests {
+
+	private final CglibSubclassingInstantiationStrategy strategy = new CglibSubclassingInstantiationStrategy();
+
+	@Nullable
+	public static Object valueToReturnFromReplacer;
+
+	@Test
+	void methodOverride() {
+		StaticListableBeanFactory beanFactory = new StaticListableBeanFactory(Map.of(
+				"myBean", new MyBean(),
+				"replacer", new MyReplacer()
+		));
+
+		RootBeanDefinition bd = new RootBeanDefinition(MyBean.class);
+		MethodOverrides methodOverrides = new MethodOverrides();
+		Stream.of("getBoolean", "getShort", "getInt", "getLong", "getFloat", "getDouble", "getByte")
+				.forEach(methodToOverride -> addOverride(methodOverrides, methodToOverride));
+		bd.setMethodOverrides(methodOverrides);
+
+		MyBean bean = (MyBean) strategy.instantiate(bd, "myBean", beanFactory);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getBoolean);
+		valueToReturnFromReplacer = true;
+		assertThat(bean.getBoolean()).isTrue();
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getShort);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getShort()).isEqualTo((short) 123);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getInt);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getInt()).isEqualTo(123);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getLong);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getLong()).isEqualTo(123L);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getFloat);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getFloat()).isEqualTo(123f);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getDouble);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getDouble()).isEqualTo(123d);
+
+		valueToReturnFromReplacer = null;
+		assertCorrectExceptionThrownBy(bean::getByte);
+		valueToReturnFromReplacer = 123;
+		assertThat(bean.getByte()).isEqualTo((byte) 123);
+	}
+
+	private void assertCorrectExceptionThrownBy(ThrowableAssert.ThrowingCallable runnable) {
+		assertThatThrownBy(runnable)
+				.isInstanceOf(NullPointerException.class)
+				.hasMessageMatching("Null return value from replacer does not match primitive return type for: "
+				                    + "\\w+ org\\.springframework\\.beans\\.factory\\.support\\.CglibSubclassingInstantiationStrategyTests\\$MyBean\\.\\w+\\(\\)");
+	}
+
+	private void addOverride(MethodOverrides methodOverrides, String methodToOverride) {
+		methodOverrides.addOverride(new ReplaceOverride(methodToOverride, "replacer"));
+	}
+
+	static class MyBean {
+		boolean getBoolean() {
+			return true;
+		}
+
+		short getShort() {
+			return 123;
+		}
+
+		int getInt() {
+			return 123;
+		}
+
+		long getLong() {
+			return 123;
+		}
+
+		float getFloat() {
+			return 123;
+		}
+
+		double getDouble() {
+			return 123;
+		}
+
+		byte getByte() {
+			return 123;
+		}
+	}
+
+	static class MyReplacer implements MethodReplacer {
+
+		@Override
+		public Object reimplement(Object obj, Method method, Object[] args) {
+			return CglibSubclassingInstantiationStrategyTests.valueToReturnFromReplacer;
+		}
+	}
+}