GH-8757: Add nullabilty to .support.converter

Fixes #8757

* Add ctor `ObjectStringMessageConverter(Charset)`
* create tests for `ObjectStringMessageConverter`
* fix codestyle issues
* revise JavaDoc
* `@NonNullApi` for `org.springframework.integration.support.converter`
* revise `SimpleMessageConverter`
* `inboundMessageMapper` and `outboundMessageMapper` are initialized to their default implementations
Bboth are set in non-default ctors with final setters
* remove `@Nullable` from `private BeanFactory beanFactory`
* Handle empty `String... headerNames`
* Some code clean up

Author: mrpiggi

spring-integration-core/src/main/java/org/springframework/integration/support/converter/AllowListDeserializingConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -30,6 +30,7 @@
 import org.springframework.core.serializer.DefaultDeserializer;
 import org.springframework.core.serializer.Deserializer;
 import org.springframework.core.serializer.support.SerializationFailedException;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.PatternMatchUtils;
 
@@ -55,6 +56,7 @@ public class AllowListDeserializingConverter implements Converter<byte[], Object
 
 	private final Deserializer<Object> deserializer;
 
+	@Nullable
 	private final ClassLoader defaultDeserializerClassLoader;
 
 	private final boolean usingDefaultDeserializer;

spring-integration-core/src/main/java/org/springframework/integration/support/converter/DefaultDatatypeChannelMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2019 the original author or authors.
+ * Copyright 2014-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,6 +22,7 @@
 import org.springframework.core.convert.ConversionService;
 import org.springframework.core.convert.support.DefaultConversionService;
 import org.springframework.integration.support.utils.IntegrationUtils;
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.MessageConverter;
@@ -33,22 +34,21 @@
  * if present.
  *
  * @author Gary Russell
+ *
  * @since 4.0
  *
  */
-public class DefaultDatatypeChannelMessageConverter implements MessageConverter,
-		BeanFactoryAware {
+public class DefaultDatatypeChannelMessageConverter implements MessageConverter, BeanFactoryAware {
 
-	private volatile ConversionService conversionService = DefaultConversionService.getSharedInstance();
+	private ConversionService conversionService = DefaultConversionService.getSharedInstance();
 
-	private volatile boolean conversionServiceSet;
+	private boolean conversionServiceSet;
 
 	/**
 	 * Specify the {@link ConversionService} to use when trying to convert to
 	 * requested type. If this property is not set explicitly but
 	 * the converter is managed within a context, it will attempt to locate a
 	 * bean named "integrationConversionService" defined within that context.
-	 *
 	 * @param conversionService The conversion service.
 	 */
 	public void setConversionService(ConversionService conversionService) {
@@ -59,7 +59,7 @@ public void setConversionService(ConversionService conversionService) {
 
 	@Override
 	public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
-		if (!this.conversionServiceSet && beanFactory != null) {
+		if (!this.conversionServiceSet) {
 			ConversionService integrationConversionService = IntegrationUtils.getConversionService(beanFactory);
 			if (integrationConversionService != null) {
 				this.conversionService = integrationConversionService;
@@ -81,7 +81,7 @@ public Object fromMessage(Message<?> message, Class<?> targetClass) {
 	}
 
 	@Override
-	public Message<?> toMessage(Object payload, MessageHeaders header) {
+	public Message<?> toMessage(Object payload, @Nullable MessageHeaders header) {
 		throw new UnsupportedOperationException("This converter does not support this method");
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/support/converter/MapMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 /**
  * Converts to/from a Map with 2 keys ('headers' and 'payload').
@@ -43,15 +44,15 @@
  */
 public class MapMessageConverter implements MessageConverter, BeanFactoryAware {
 
-	private volatile String[] headerNames;
+	private String[] headerNames = {};
 
-	private volatile boolean filterHeadersInToMessage;
+	private boolean filterHeadersInToMessage;
 
-	private volatile BeanFactory beanFactory;
+	private BeanFactory beanFactory;
 
-	private volatile MessageBuilderFactory messageBuilderFactory = new DefaultMessageBuilderFactory();
+	private MessageBuilderFactory messageBuilderFactory = new DefaultMessageBuilderFactory();
 
-	private volatile boolean messageBuilderFactorySet;
+	private boolean messageBuilderFactorySet;
 
 
 	@Override
@@ -77,16 +78,19 @@ protected MessageBuilderFactory getMessageBuilderFactory() {
 	 * @param headerNames The header names.
 	 */
 	public void setHeaderNames(String... headerNames) {
-		Assert.notEmpty(headerNames, "at least one header name is required");
-		this.headerNames = Arrays.copyOf(headerNames, headerNames.length);
+		if (ObjectUtils.isEmpty(headerNames)) {
+			this.headerNames = new String[] {};
+		}
+		else {
+			this.headerNames = Arrays.copyOf(headerNames, headerNames.length);
+		}
 	}
 
 	/**
-	 * By default all headers on Map passed to {@link #toMessage(Object, MessageHeaders)}
+	 * By default, all headers on Map passed to {@link #toMessage(Object, MessageHeaders)}
 	 * will be mapped. Set this property
-	 * to 'true' if you wish to limit the inbound headers to those in
-	 * the #headerNames.
-	 *
+	 * to 'true' if you wish to limit the inbound headers to those
+	 * in the {@link #headerNames}.
 	 * @param filterHeadersInToMessage true if the headers should be filtered.
 	 */
 	public void setFilterHeadersInToMessage(boolean filterHeadersInToMessage) {

spring-integration-core/src/main/java/org/springframework/integration/support/converter/ObjectStringMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 the original author or authors.
+ * Copyright 2017-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,6 +16,9 @@
 
 package org.springframework.integration.support.converter;
 
+import java.nio.charset.Charset;
+
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.converter.StringMessageConverter;
 
@@ -24,16 +27,31 @@
  * <p>
  * Delegates to super when payload is {@code byte[]} or {@code String}.
  * Performs {@link Object#toString()} in other cases.
+ * <p>
+ * This class is intended to serve as a fallback converter for internal message deserialization purposes. Therefore, it
+ * is recommended to exclusively use the
+ * {@link org.springframework.messaging.converter.AbstractMessageConverter#fromMessage(Message, Class) fromMessage}
+ * method with {@code String.class} as the {@code targetClass}.
  *
  * @author Marius Bogoevici
  * @author Artem Bilan
+ * @author Falk Hanisch
  *
  * @since 5.0
  */
 public class ObjectStringMessageConverter extends StringMessageConverter {
 
+	public ObjectStringMessageConverter(Charset defaultCharset) {
+		super(defaultCharset);
+	}
+
+	public ObjectStringMessageConverter() {
+		super();
+	}
+
+
 	@Override
-	protected Object convertFromInternal(Message<?> message, Class<?> targetClass, Object conversionHint) {
+	protected Object convertFromInternal(Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
 		Object payload = message.getPayload();
 		if (payload instanceof String || payload instanceof byte[]) {
 			return super.convertFromInternal(message, targetClass, conversionHint);

spring-integration-core/src/main/java/org/springframework/integration/support/converter/PassThruMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2019 the original author or authors.
+ * Copyright 2015-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,11 +16,11 @@
 
 package org.springframework.integration.support.converter;
 
+import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.support.MessageBuilder;
-import org.springframework.util.Assert;
 
 /**
  * The simple {@link MessageConverter} implementation which contract is to return
@@ -31,6 +31,7 @@
  * operates only with {@link Message}s, e.g. Spring Integration Adapters.
  *
  * @author Artem Bilan
+ *
  * @since 4.2
  */
 public class PassThruMessageConverter implements MessageConverter {
@@ -41,9 +42,13 @@ public Object fromMessage(Message<?> message, Class<?> targetClass) {
 	}
 
 	@Override
-	public Message<?> toMessage(Object payload, MessageHeaders headers) {
-		Assert.isInstanceOf(byte[].class, payload, "'payload' must be of 'byte[]' type.");
-		return MessageBuilder.createMessage(payload, headers);
+	public Message<?> toMessage(Object payload, @Nullable MessageHeaders headers) {
+		if (payload instanceof byte[]) {
+			return MessageBuilder.withPayload(payload).copyHeaders(headers).build();
+		}
+		else {
+			return null;
+		}
 	}
 
 }

spring-integration-core/src/main/java/org/springframework/integration/support/converter/SimpleMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,51 +42,50 @@
 @SuppressWarnings({"unchecked", "rawtypes"})
 public class SimpleMessageConverter implements MessageConverter, BeanFactoryAware {
 
-	private volatile InboundMessageMapper inboundMessageMapper;
+	private InboundMessageMapper inboundMessageMapper = new DefaultInboundMessageMapper();
 
-	private volatile OutboundMessageMapper outboundMessageMapper;
+	private OutboundMessageMapper outboundMessageMapper = new DefaultOutboundMessageMapper();
 
-	private volatile MessageBuilderFactory messageBuilderFactory = new DefaultMessageBuilderFactory();
+	private MessageBuilderFactory messageBuilderFactory = new DefaultMessageBuilderFactory();
 
-	private volatile boolean messageBuilderFactorySet;
+	private boolean messageBuilderFactorySet;
 
 	private BeanFactory beanFactory;
 
-	public SimpleMessageConverter() {
-		this(null, null);
-	}
+	public SimpleMessageConverter() { }
 
 	public SimpleMessageConverter(InboundMessageMapper<?> inboundMessageMapper) {
-		this(inboundMessageMapper,
-				(inboundMessageMapper instanceof OutboundMessageMapper
-						? (OutboundMessageMapper<?>) inboundMessageMapper
-						: null));
+		setInboundMessageMapper(inboundMessageMapper);
+		if (inboundMessageMapper instanceof OutboundMessageMapper<?> messageMapper) {
+			setOutboundMessageMapper(messageMapper);
+		}
 	}
 
 	public SimpleMessageConverter(OutboundMessageMapper<?> outboundMessageMapper) {
-		this(outboundMessageMapper instanceof InboundMessageMapper
-						? (InboundMessageMapper<?>) outboundMessageMapper
-						: null,
-				outboundMessageMapper);
+		if (outboundMessageMapper instanceof InboundMessageMapper<?> messageMapper) {
+			setInboundMessageMapper(messageMapper);
+		}
+		setOutboundMessageMapper(outboundMessageMapper);
 	}
 
 	public SimpleMessageConverter(InboundMessageMapper<?> inboundMessageMapper,
 			OutboundMessageMapper<?> outboundMessageMapper) {
-		this.setInboundMessageMapper(inboundMessageMapper);
-		this.setOutboundMessageMapper(outboundMessageMapper);
+
+		setInboundMessageMapper(inboundMessageMapper);
+		setOutboundMessageMapper(outboundMessageMapper);
 	}
 
 
-	public final void setInboundMessageMapper(InboundMessageMapper<?> inboundMessageMapper) {
-		this.inboundMessageMapper = (inboundMessageMapper != null)
-				? inboundMessageMapper
-				: new DefaultInboundMessageMapper();
+	public final void setInboundMessageMapper(@Nullable InboundMessageMapper<?> inboundMessageMapper) {
+		if (inboundMessageMapper != null) {
+			this.inboundMessageMapper = inboundMessageMapper;
+		}
 	}
 
-	public final void setOutboundMessageMapper(OutboundMessageMapper<?> outboundMessageMapper) {
-		this.outboundMessageMapper = (outboundMessageMapper != null
-				? outboundMessageMapper
-				: new DefaultOutboundMessageMapper());
+	public final void setOutboundMessageMapper(@Nullable OutboundMessageMapper<?> outboundMessageMapper) {
+		if (outboundMessageMapper != null) {
+			this.outboundMessageMapper = outboundMessageMapper;
+		}
 	}
 
 	@Override
@@ -110,8 +109,8 @@ public Message<?> toMessage(Object object, @Nullable MessageHeaders headers) {
 		try {
 			return this.inboundMessageMapper.toMessage(object, headers);
 		}
-		catch (Exception e) {
-			throw new MessageConversionException("failed to convert object to Message", e);
+		catch (Exception ex) {
+			throw new MessageConversionException("failed to convert object to Message", ex);
 		}
 	}
 
@@ -121,8 +120,8 @@ public Object fromMessage(Message<?> message, Class<?> targetClass) {
 		try {
 			return this.outboundMessageMapper.fromMessage(message);
 		}
-		catch (Exception e) {
-			throw new MessageConversionException(message, "failed to convert Message to object", e);
+		catch (Exception ex) {
+			throw new MessageConversionException(message, "failed to convert Message to object", ex);
 		}
 	}
 

spring-integration-core/src/main/java/org/springframework/integration/support/converter/package-info.java

@@ -1,4 +1,6 @@
 /**
  * Provides classes supporting message conversion.
  */
+@org.springframework.lang.NonNullApi
+@org.springframework.lang.NonNullFields
 package org.springframework.integration.support.converter;