Make getSharedInstance() unmodifiable

Update `ApplicationConversionService.getSharedInstance()` so that the
instance returned is unmodifiable and converters cannot be added or
removed from it.

Closes gh-26088

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -60,8 +60,6 @@ import org.springframework.context.support.AbstractApplicationContext;
 import org.springframework.context.support.GenericApplicationContext;
 import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
-import org.springframework.core.convert.ConversionService;
-import org.springframework.core.convert.support.ConfigurableConversionService;
 import org.springframework.core.env.CommandLinePropertySource;
 import org.springframework.core.env.CompositePropertySource;
 import org.springframework.core.env.ConfigurableEnvironment;
@@ -512,8 +510,7 @@ public class SpringApplication {
 	 */
 	protected void configureEnvironment(ConfigurableEnvironment environment, String[] args) {
 		if (this.addConversionService) {
-			ConversionService conversionService = ApplicationConversionService.getSharedInstance();
-			environment.setConversionService((ConfigurableConversionService) conversionService);
+			environment.setConversionService(new ApplicationConversionService());
 		}
 		configurePropertySources(environment, args);
 		configureProfiles(environment, args);
@@ -633,7 +630,7 @@ public class SpringApplication {
 			}
 		}
 		if (this.addConversionService) {
-			context.getBeanFactory().setConversionService(ApplicationConversionService.getSharedInstance());
+			context.getBeanFactory().setConversionService(context.getEnvironment().getConversionService());
 		}
 	}
 

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/ApplicationConversionService.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.convert;
 
+import java.lang.annotation.Annotation;
 import java.util.LinkedHashSet;
 import java.util.Set;
 
@@ -23,11 +24,13 @@ import org.springframework.beans.factory.ListableBeanFactory;
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.TypeDescriptor;
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.core.convert.converter.ConverterFactory;
 import org.springframework.core.convert.converter.ConverterRegistry;
 import org.springframework.core.convert.converter.GenericConverter;
 import org.springframework.core.convert.converter.GenericConverter.ConvertiblePair;
 import org.springframework.core.convert.support.ConfigurableConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.format.AnnotationFormatterFactory;
 import org.springframework.format.Formatter;
 import org.springframework.format.FormatterRegistry;
 import org.springframework.format.Parser;
@@ -52,15 +55,96 @@ public class ApplicationConversionService extends FormattingConversionService {
 
 	private static volatile ApplicationConversionService sharedInstance;
 
+	private boolean unmodifiable;
+
 	public ApplicationConversionService() {
 		this(null);
 	}
 
 	public ApplicationConversionService(StringValueResolver embeddedValueResolver) {
+		this(embeddedValueResolver, false);
+	}
+
+	private ApplicationConversionService(StringValueResolver embeddedValueResolver, boolean unmodifiable) {
 		if (embeddedValueResolver != null) {
 			setEmbeddedValueResolver(embeddedValueResolver);
 		}
 		configure(this);
+		this.unmodifiable = unmodifiable;
+	}
+
+	@Override
+	public void addPrinter(Printer<?> printer) {
+		assertModifiable();
+		super.addPrinter(printer);
+	}
+
+	@Override
+	public void addParser(Parser<?> parser) {
+		assertModifiable();
+		super.addParser(parser);
+	}
+
+	@Override
+	public void addFormatter(Formatter<?> formatter) {
+		assertModifiable();
+		super.addFormatter(formatter);
+	}
+
+	@Override
+	public void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter) {
+		assertModifiable();
+		super.addFormatterForFieldType(fieldType, formatter);
+	}
+
+	@Override
+	public void addConverter(Converter<?, ?> converter) {
+		assertModifiable();
+		super.addConverter(converter);
+	}
+
+	@Override
+	public void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser) {
+		assertModifiable();
+		super.addFormatterForFieldType(fieldType, printer, parser);
+	}
+
+	@Override
+	public void addFormatterForFieldAnnotation(
+			AnnotationFormatterFactory<? extends Annotation> annotationFormatterFactory) {
+		assertModifiable();
+		super.addFormatterForFieldAnnotation(annotationFormatterFactory);
+	}
+
+	@Override
+	public <S, T> void addConverter(Class<S> sourceType, Class<T> targetType,
+			Converter<? super S, ? extends T> converter) {
+		assertModifiable();
+		super.addConverter(sourceType, targetType, converter);
+	}
+
+	@Override
+	public void addConverter(GenericConverter converter) {
+		assertModifiable();
+		super.addConverter(converter);
+	}
+
+	@Override
+	public void addConverterFactory(ConverterFactory<?, ?> factory) {
+		assertModifiable();
+		super.addConverterFactory(factory);
+	}
+
+	@Override
+	public void removeConvertible(Class<?> sourceType, Class<?> targetType) {
+		assertModifiable();
+		super.removeConvertible(sourceType, targetType);
+	}
+
+	private void assertModifiable() {
+		if (this.unmodifiable) {
+			throw new UnsupportedOperationException("This ApplicationConversionService cannot be modified");
+		}
 	}
 
 	/**
@@ -101,7 +185,7 @@ public class ApplicationConversionService extends FormattingConversionService {
 			synchronized (ApplicationConversionService.class) {
 				sharedInstance = ApplicationConversionService.sharedInstance;
 				if (sharedInstance == null) {
-					sharedInstance = new ApplicationConversionService();
+					sharedInstance = new ApplicationConversionService(null, true);
 					ApplicationConversionService.sharedInstance = sharedInstance;
 				}
 			}

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/convert/ApplicationConversionServiceTests.java

@@ -21,6 +21,7 @@ import java.util.List;
 import java.util.Locale;
 import java.util.Set;
 
+import org.assertj.core.api.ThrowableAssert.ThrowingCallable;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.context.ConfigurableApplicationContext;
@@ -34,6 +35,7 @@ import org.springframework.format.Parser;
 import org.springframework.format.Printer;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.verifyNoMoreInteractions;
@@ -113,6 +115,28 @@ class ApplicationConversionServiceTests {
 		assertThat(conversionService.isConvertViaObjectSourceType(sourceType, targetType)).isFalse();
 	}
 
+	@Test
+	void sharedInstanceCannotBeModified() {
+		ApplicationConversionService instance = (ApplicationConversionService) ApplicationConversionService
+				.getSharedInstance();
+		assertUnmodifiableExceptionThrown(() -> instance.addPrinter(null));
+		assertUnmodifiableExceptionThrown(() -> instance.addParser(null));
+		assertUnmodifiableExceptionThrown(() -> instance.addFormatter(null));
+		assertUnmodifiableExceptionThrown(() -> instance.addFormatterForFieldType(null, null));
+		assertUnmodifiableExceptionThrown(() -> instance.addConverter((Converter<?, ?>) null));
+		assertUnmodifiableExceptionThrown(() -> instance.addFormatterForFieldType(null, null, null));
+		assertUnmodifiableExceptionThrown(() -> instance.addFormatterForFieldAnnotation(null));
+		assertUnmodifiableExceptionThrown(() -> instance.addConverter(null, null, null));
+		assertUnmodifiableExceptionThrown(() -> instance.addConverter((GenericConverter) null));
+		assertUnmodifiableExceptionThrown(() -> instance.addConverterFactory(null));
+		assertUnmodifiableExceptionThrown(() -> instance.removeConvertible(null, null));
+	}
+
+	private void assertUnmodifiableExceptionThrown(ThrowingCallable throwingCallable) {
+		assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(throwingCallable)
+				.withMessage("This ApplicationConversionService cannot be modified");
+	}
+
 	static class ExampleGenericConverter implements GenericConverter {
 
 		@Override