Use Class reference rather than String for customizer

Update `StructuredLoggingJsonProperties` to use a real Class reference
rather than a String.

Closes gh-43202

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/structured/StructuredLoggingJsonProperties.java

@@ -34,7 +34,7 @@ import org.springframework.core.env.Environment;
  * @author Phillip Webb
  */
 record StructuredLoggingJsonProperties(Set<String> include, Set<String> exclude, Map<String, String> rename,
-		Map<String, String> add, String customizer) {
+		Map<String, String> add, Class<? extends StructureLoggingJsonMembersCustomizer<?>> customizer) {
 
 	static StructuredLoggingJsonProperties get(Environment environment) {
 		return Binder.get(environment)

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/structured/StructuredLoggingJsonPropertiesJsonMembersCustomizer.java

@@ -22,7 +22,6 @@ import org.springframework.boot.json.JsonWriter.MemberPath;
 import org.springframework.boot.json.JsonWriter.Members;
 import org.springframework.boot.util.Instantiator;
 import org.springframework.util.CollectionUtils;
-import org.springframework.util.StringUtils;
 
 /**
  * {@link StructureLoggingJsonMembersCustomizer} to apply
@@ -50,8 +49,8 @@ class StructuredLoggingJsonPropertiesJsonMembersCustomizer implements StructureL
 		if (!CollectionUtils.isEmpty(add)) {
 			add.forEach(members::add);
 		}
-		String customizer = this.properties.customizer();
-		if (StringUtils.hasLength(customizer)) {
+		Class<? extends StructureLoggingJsonMembersCustomizer<?>> customizer = this.properties.customizer();
+		if (customizer != null) {
 			createAndApplyCustomizer(members, customizer);
 		}
 	}
@@ -71,8 +70,9 @@ class StructuredLoggingJsonPropertiesJsonMembersCustomizer implements StructureL
 	}
 
 	@SuppressWarnings({ "unchecked", "rawtypes" })
-	private void createAndApplyCustomizer(Members<Object> members, String customizerClassName) {
-		((StructureLoggingJsonMembersCustomizer) this.instantiator.instantiate(customizerClassName)).customize(members);
+	private void createAndApplyCustomizer(Members<Object> members,
+			Class<? extends StructureLoggingJsonMembersCustomizer<?>> customizerClass) {
+		((StructureLoggingJsonMembersCustomizer) this.instantiator.instantiateType(customizerClass)).customize(members);
 	}
 
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/util/Instantiator.java

@@ -148,6 +148,17 @@ public class Instantiator<T> {
 		return instantiate(TypeSupplier.forName(classLoader, name));
 	}
 
+	/**
+	 * Instantiate the given set of classes, injecting constructor arguments as necessary.
+	 * @param type the types to instantiate
+	 * @return a list of instantiated instances
+	 * @since 3.4.0
+	 */
+	public T instantiateType(Class<?> type) {
+		Assert.notNull(type, "Type must not be null");
+		return instantiate(TypeSupplier.forType(type));
+	}
+
 	/**
 	 * Instantiate the given set of classes, injecting constructor arguments as necessary.
 	 * @param types the types to instantiate

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/structured/StructuredLoggingJsonPropertiesJsonMembersCustomizerTests.java

@@ -26,6 +26,7 @@ import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 
 import org.springframework.boot.json.JsonWriter;
+import org.springframework.boot.json.JsonWriter.Members;
 import org.springframework.boot.json.JsonWriter.NameProcessor;
 import org.springframework.boot.util.Instantiator;
 
@@ -95,13 +96,13 @@ class StructuredLoggingJsonPropertiesJsonMembersCustomizerTests {
 	}
 
 	@Test
-	@SuppressWarnings("rawtypes")
+	@SuppressWarnings({ "rawtypes", "unchecked" })
 	void customizeWhenHasCustomizerCustomizesMember() {
 		StructureLoggingJsonMembersCustomizer<?> uppercaseCustomizer = (members) -> members
 			.applyingNameProcessor(NameProcessor.of(String::toUpperCase));
-		given(((Instantiator) this.instantiator).instantiate("test")).willReturn(uppercaseCustomizer);
+		given(((Instantiator) this.instantiator).instantiateType(TestCustomizer.class)).willReturn(uppercaseCustomizer);
 		StructuredLoggingJsonProperties properties = new StructuredLoggingJsonProperties(Collections.emptySet(),
-				Collections.emptySet(), Collections.emptyMap(), Collections.emptyMap(), "test");
+				Collections.emptySet(), Collections.emptyMap(), Collections.emptyMap(), TestCustomizer.class);
 		StructuredLoggingJsonPropertiesJsonMembersCustomizer customizer = new StructuredLoggingJsonPropertiesJsonMembersCustomizer(
 				this.instantiator, properties);
 		assertThat(writeSampleJson(customizer)).contains("\"A\":\"a\"");
@@ -117,4 +118,12 @@ class StructuredLoggingJsonPropertiesJsonMembersCustomizerTests {
 		}).writeToString(new Object());
 	}
 
+	static class TestCustomizer implements StructureLoggingJsonMembersCustomizer<String> {
+
+		@Override
+		public void customize(Members<String> members) {
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/structured/StructuredLoggingJsonPropertiesTests.java

@@ -21,6 +21,7 @@ import java.util.Set;
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.boot.json.JsonWriter.Members;
 import org.springframework.mock.env.MockEnvironment;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -39,10 +40,10 @@ class StructuredLoggingJsonPropertiesTests {
 		environment.setProperty("logging.structured.json.exclude", "c,d");
 		environment.setProperty("logging.structured.json.rename.e", "f");
 		environment.setProperty("logging.structured.json.add.g", "h");
-		environment.setProperty("logging.structured.json.customizer", "i");
+		environment.setProperty("logging.structured.json.customizer", TestCustomizer.class.getName());
 		StructuredLoggingJsonProperties properties = StructuredLoggingJsonProperties.get(environment);
 		assertThat(properties).isEqualTo(new StructuredLoggingJsonProperties(Set.of("a", "b"), Set.of("c", "d"),
-				Map.of("e", "f"), Map.of("g", "h"), "i"));
+				Map.of("e", "f"), Map.of("g", "h"), TestCustomizer.class));
 	}
 
 	@Test
@@ -51,4 +52,12 @@ class StructuredLoggingJsonPropertiesTests {
 		StructuredLoggingJsonProperties.get(environment);
 	}
 
+	static class TestCustomizer implements StructureLoggingJsonMembersCustomizer<String> {
+
+		@Override
+		public void customize(Members<String> members) {
+		}
+
+	}
+
 }