Spring-MessageConverter

README.md

@@ -56,7 +56,8 @@
 
 ### Spring Message
 - [Spring EnableJMS](/docs/Spring/message/Spring-EnableJms.md)
-
+- [Spring JmsTemplate](/docs/Spring/message/Spring-JmsTemplate.md)
+- [Spring MessageConverter](/docs/Spring/message/Spring-MessageConverter.md)
 ## MyBatis
 ### 基础支持层
 - [反射工具箱和 TypeHandler 系列](docs/Mybatis/基础支持层/1、反射工具箱和TypeHandler系列.md)

docs/Spring/message/Spring-MessageConverter.md

@@ -0,0 +1,197 @@
+# Spring MessageConverter
+- Author: [HuiFer](https://github.com/huifer)
+- 源码阅读仓库: [SourceHot-spring](https://github.com/SourceHot/spring-framework-read)
+- 源码路径: `org.springframework.messaging.converter.MessageConverter`
+## MessageConverter
+- 消息转换接口
+- 类图如下
+![image-20200305085013723](/images/springmessage/image-20200305085013723.png)
+- 两个方法
+1. fromMessage: 从消息转换到Object
+```java
+    	Object fromMessage(Message<?> message, Class<?> targetClass);
+```
+2. toMessage: 从Object转换到消息
+```java
+    	Message<?> toMessage(Object payload, @Nullable MessageHeaders headers);
+```
+
+
+
+
+
+| 序号 | class                           | 作用                 |
+| ---- | ------------------------------- | -------------------- |
+| 1    | ByteArrayMessageConverter       | byte数组消息转换器   |
+| 2    | MappingJackson2MessageConverter | jackson2的消息转换器 |
+| 3    | MarshallingMessageConverter     | xml的消息转换器      |
+| 4    | StringMessageConverter          | 字符串消息转换器     |
+
+
+
+
+
+## AbstractMessageConverter
+
+类图: 
+
+![image-20200305085845017](/images/springmessage/image-20200305085845017.png)
+
+### fromMessage
+
+```java
+	@Override
+	@Nullable
+	public final Object fromMessage(Message<?> message, Class<?> targetClass) {
+		return fromMessage(message, targetClass, null);
+	}
+
+	@Override
+	@Nullable
+	public final Object fromMessage(Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
+		if (!canConvertFrom(message, targetClass)) {
+			return null;
+		}
+		return convertFromInternal(message, targetClass, conversionHint);
+	}
+
+
+	// 子类实现
+	@Nullable
+	protected Object convertFromInternal(
+			Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
+
+		return null;
+	}
+
+```
+
+- 真正的转换过程
+
+```JAVA
+	@Override
+	protected Object convertFromInternal(Message<?> message, Class<?> targetClass, @Nullable Object conversionHint) {
+		Charset charset = getContentTypeCharset(getMimeType(message.getHeaders()));
+		Object payload = message.getPayload();
+		return (payload instanceof String ? payload : new String((byte[]) payload, charset));
+	}
+
+```
+
+
+
+### toMessage
+
+```JAVA
+    @Override
+    @Nullable
+    public final Message<?> toMessage(Object payload, @Nullable MessageHeaders headers) {
+        return toMessage(payload, headers, null);
+    }
+
+```
+
+
+
+```JAVA
+    @Override
+    @Nullable
+    public final Message<?> toMessage(Object payload, @Nullable MessageHeaders headers, @Nullable Object conversionHint) {
+        if (!canConvertTo(payload, headers)) {
+            return null;
+        }
+        // 子类实现
+        Object payloadToUse = convertToInternal(payload, headers, conversionHint);
+        if (payloadToUse == null) {
+            return null;
+        }
+
+        MimeType mimeType = getDefaultContentType(payloadToUse);
+        if (headers != null) {
+            MessageHeaderAccessor accessor = MessageHeaderAccessor.getAccessor(headers, MessageHeaderAccessor.class);
+            if (accessor != null && accessor.isMutable()) {
+                if (mimeType != null) {
+                    accessor.setHeaderIfAbsent(MessageHeaders.CONTENT_TYPE, mimeType);
+                }
+                // 创建信息对象
+                return MessageBuilder.createMessage(payloadToUse, accessor.getMessageHeaders());
+            }
+        }
+
+        MessageBuilder<?> builder = MessageBuilder.withPayload(payloadToUse);
+        if (headers != null) {
+            builder.copyHeaders(headers);
+        }
+        if (mimeType != null) {
+            builder.setHeaderIfAbsent(MessageHeaders.CONTENT_TYPE, mimeType);
+        }
+        return builder.build();
+    }
+
+
+```
+
+- `org.springframework.messaging.converter.StringMessageConverter#convertToInternal`
+
+  ```JAVA
+  	@Override
+  	@Nullable
+  	protected Object convertToInternal(
+  			Object payload, @Nullable MessageHeaders headers, @Nullable Object conversionHint) {
+  
+  		if (byte[].class == getSerializedPayloadClass()) {
+  			// 获取编码
+  			Charset charset = getContentTypeCharset(getMimeType(headers));
+  			// 获取byte数组
+  			payload = ((String) payload).getBytes(charset);
+  		}
+  		return payload;
+  	}
+  
+  ```
+
+  
+
+- 创建**Message**对象
+
+  ```JAVA
+  	@SuppressWarnings("unchecked")
+  	public static <T> Message<T> createMessage(@Nullable T payload, MessageHeaders messageHeaders) {
+  		Assert.notNull(payload, "Payload must not be null");
+  		Assert.notNull(messageHeaders, "MessageHeaders must not be null");
+  		if (payload instanceof Throwable) {
+  			return (Message<T>) new ErrorMessage((Throwable) payload, messageHeaders);
+  		}
+  		else {
+  			return new GenericMessage<>(payload, messageHeaders);
+  		}
+  	}
+  
+  ```
+
+  
+
+  ```JAVA
+  	@SuppressWarnings("unchecked")
+  	public Message<T> build() {
+  		if (this.originalMessage != null && !this.headerAccessor.isModified()) {
+  			return this.originalMessage;
+  		}
+  		MessageHeaders headersToUse = this.headerAccessor.toMessageHeaders();
+  		if (this.payload instanceof Throwable) {
+  			return (Message<T>) new ErrorMessage((Throwable) this.payload, headersToUse);
+  		}
+  		else {
+  			return new GenericMessage<>(this.payload, headersToUse);
+  		}
+  	}
+  
+  ```
+
+  
+
+- 两种创建方式基本相同，如果出现异常组装异常消息对象`ErrorMessage`,成功创建`GenericMessage`
+
+![image-20200305090846313](/images/springmessage/image-20200305090846313.png)
+
+从类图上看`ErrorMessage`是`GenericMessage`的子类