转载spring restemplate

什么是RestTemplate？

RestTemplate是Spring提供的用于访问Rest服务的客户端，RestTemplate提供了多种便捷访问远程Http服务的方法，能够大大提高客户端的编写效率。 
调用RestTemplate的默认构造函数，RestTemplate对象在底层通过使用java.net包下的实现创建HTTP 请求，可以通过使用ClientHttpRequestFactory指定不同的HTTP请求方式。ClientHttpRequestFactory接口主要提供了两种实现方式

一种是SimpleClientHttpRequestFactory，使用J2SE提供的方式（既java.net包提供的方式）创建底层的Http请求连接。 
一种方式是使用HttpComponentsClientHttpRequestFactory方式，底层使用HttpClient访问远程的Http服务，使用HttpClient可以配置连接池和证书等信息。

xml配置的方式

请查看RestTemplate源码了解细节，知其然知其所以然！

RestTemplate默认是使用SimpleClientHttpRequestFactory，内部是调用jdk的HttpConnection，默认超时为-1

基于jdk的spring的RestTemplate

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"
       default-autowire="byName" default-lazy-init="true">

    <!--方式一、使用jdk的实现-->
    <bean id="ky.requestFactory" class="org.springframework.http.client.SimpleClientHttpRequestFactory">
        <property name="readTimeout" value="10000"/>
        <property name="connectTimeout" value="5000"/>
    </bean>

    <bean id="simpleRestTemplate" class="org.springframework.web.client.RestTemplate">
        <constructor-arg ref="ky.requestFactory"/>
        <property name="messageConverters">
            <list>
                <bean class="org.springframework.http.converter.FormHttpMessageConverter"/>
                <bean class="org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter"/>
                <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"/>
                <bean class="org.springframework.http.converter.StringHttpMessageConverter">
                    <property name="supportedMediaTypes">
                        <list>
                            <value>text/plain;charset=UTF-8</value>
                        </list>
                    </property>
                </bean>
            </list>
        </property>
    </bean>

</beans>

使用Httpclient连接池的方式

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"
       default-autowire="byName" default-lazy-init="true">

    <!--方式二、使用httpclient的实现，带连接池-->
    <bean id="ky.pollingConnectionManager" class="org.apache.http.impl.conn.PoolingHttpClientConnectionManager">
        <!--整个连接池的并发-->
        <property name="maxTotal" value="1000" />
        <!--每个主机的并发-->
        <property name="defaultMaxPerRoute" value="1000" />
    </bean>

    <bean id="ky.httpClientBuilder" class="org.apache.http.impl.client.HttpClientBuilder" factory-method="create">
        <property name="connectionManager" ref="ky.pollingConnectionManager" />
        <!--开启重试-->
        <property name="retryHandler">
            <bean class="org.apache.http.impl.client.DefaultHttpRequestRetryHandler">
                <constructor-arg value="2"/>
                <constructor-arg value="true"/>
            </bean>
        </property>
        <property name="defaultHeaders">
            <list>
                <bean class="org.apache.http.message.BasicHeader">
                    <constructor-arg value="User-Agent"/>
                    <constructor-arg value="Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.16 Safari/537.36"/>
                </bean>
                <bean class="org.apache.http.message.BasicHeader">
                    <constructor-arg value="Accept-Encoding"/>
                    <constructor-arg value="gzip,deflate"/>
                </bean>
                <bean class="org.apache.http.message.BasicHeader">
                    <constructor-arg value="Accept-Language"/>
                    <constructor-arg value="zh-CN"/>
                </bean>
            </list>
        </property>
    </bean>

    <bean id="ky.httpClient" factory-bean="ky.httpClientBuilder" factory-method="build" />

    <bean id="ky.clientHttpRequestFactory" class="org.springframework.http.client.HttpComponentsClientHttpRequestFactory">
        <constructor-arg ref="ky.httpClient"/>
        <!--连接超时时间，毫秒-->
        <property name="connectTimeout" value="5000"/>
        <!--读写超时时间，毫秒-->
        <property name="readTimeout" value="10000"/>
    </bean>

    <bean id="restTemplate" class="org.springframework.web.client.RestTemplate">
        <constructor-arg ref="ky.clientHttpRequestFactory"/>
        <property name="errorHandler">
            <bean class="org.springframework.web.client.DefaultResponseErrorHandler"/>
        </property>
        <property name="messageConverters">
            <list>
                <bean class="org.springframework.http.converter.FormHttpMessageConverter"/>
                <bean class="org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter"/>
                <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"/>
                <bean class="org.springframework.http.converter.StringHttpMessageConverter">
                    <property name="supportedMediaTypes">
                        <list>
                            <value>text/plain;charset=UTF-8</value>
                        </list>
                    </property>
                </bean>
            </list>
        </property>
    </bean>

</beans>

bean初始化+静态工具

线程安全的单例（懒汉模式）

基于jdk的spring的RestTemplate

@Component
@Lazy(false)
public class SimpleRestClient {

    private static final Logger LOGGER = LoggerFactory.getLogger(SimpleRestClient.class);

    private static RestTemplate restTemplate;

    static {
        SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
        requestFactory.setReadTimeout(5000);
        requestFactory.setConnectTimeout(5000);

        // 添加转换器
        List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
        messageConverters.add(new StringHttpMessageConverter(Charset.forName("UTF-8")));
        messageConverters.add(new FormHttpMessageConverter());
        messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
        messageConverters.add(new MappingJackson2HttpMessageConverter());

        restTemplate = new RestTemplate(messageConverters);
        restTemplate.setRequestFactory(requestFactory);
        restTemplate.setErrorHandler(new DefaultResponseErrorHandler());

        LOGGER.info("SimpleRestClient初始化完成");
    }

    private SimpleRestClient() {

    }

    @PostConstruct
    public static RestTemplate getClient() {
        return restTemplate;
    }

}

使用Httpclient连接池的方式

public class RestClient {

    private static final Logger LOGGER = LoggerFactory.getLogger(SimpleRestClient.class);

    private static RestTemplate restTemplate;

    static {
        // 长连接保持30秒
        PoolingHttpClientConnectionManager pollingConnectionManager = new PoolingHttpClientConnectionManager(30, TimeUnit.SECONDS);
        // 总连接数
        pollingConnectionManager.setMaxTotal(500);
        // 同路由的并发数
        pollingConnectionManager.setDefaultMaxPerRoute(500);

        HttpClientBuilder httpClientBuilder = HttpClients.custom();
        httpClientBuilder.setConnectionManager(pollingConnectionManager);
        // 重试次数，默认是3次，没有开启
        httpClientBuilder.setRetryHandler(new DefaultHttpRequestRetryHandler(2, true));
        // 保持长连接配置，需要在头添加Keep-Alive
        httpClientBuilder.setKeepAliveStrategy(DefaultConnectionKeepAliveStrategy.INSTANCE);

        List<Header> headers = new ArrayList<>();
        headers.add(new BasicHeader("User-Agent", "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1650.16 Safari/537.36"));
        headers.add(new BasicHeader("Accept-Encoding", "gzip,deflate"));
        headers.add(new BasicHeader("Accept-Language", "zh-CN,zh;q=0.8,en;q=0.6"));
        headers.add(new BasicHeader("Connection", "keep-alive"));

        httpClientBuilder.setDefaultHeaders(headers);

        HttpClient httpClient = httpClientBuilder.build();

        // httpClient连接配置，底层是配置RequestConfig
        HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
        // 连接超时
        clientHttpRequestFactory.setConnectTimeout(5000);
        // 数据读取超时时间，即SocketTimeout
        clientHttpRequestFactory.setReadTimeout(5000);
        // 连接不够用的等待时间，不宜过长，必须设置，比如连接不够用时，时间过长将是灾难性的
        clientHttpRequestFactory.setConnectionRequestTimeout(200);
        // 缓冲请求数据，默认值是true。通过POST或者PUT大量发送数据时，建议将此属性更改为false，以免耗尽内存。
        // clientHttpRequestFactory.setBufferRequestBody(false);

        // 添加内容转换器
        List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
        messageConverters.add(new StringHttpMessageConverter(Charset.forName("UTF-8")));
        messageConverters.add(new FormHttpMessageConverter());
        messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
        messageConverters.add(new MappingJackson2HttpMessageConverter());
        messageConverters.add(new ByteArrayHttpMessageConverter());

        restTemplate = new RestTemplate(messageConverters);
        restTemplate.setRequestFactory(clientHttpRequestFactory);
        restTemplate.setErrorHandler(new DefaultResponseErrorHandler());

        LOGGER.info("RestClient初始化完成");
    }

    private RestClient() {

    }

    public static RestTemplate getClient() {
        return restTemplate;
    }

}

使用样例

注意点 
api里面可以做自动的参数匹配： 
如：http://you domainn name/test?empNo={empNo}，则下面方法的最后一个参数为数据匹配参数，会自动根据key进行查找，然后替换

API没有声明异常，注意进行异常处理

更多使用语法请查看API文档

完整的实例代码 
定义一个异常

public class RestClientException extends NestedRuntimeException {

    /**
     * 状态码
     */
    private HttpStatus statusCode;
    /**
     * 状态码文本
     */
    private String statusText;
    /**
     * 异常时返回的内容
     */
    private String responseBody;
    /**
     * 返回的头
     */
    private HttpHeaders responseHeaders;

    public RestClientException(Exception exception) {
        super(exception.getMessage(), exception);

        if (exception instanceof HttpServerErrorException) {
            HttpServerErrorException e = (HttpServerErrorException) exception;

            this.statusCode = e.getStatusCode();
            this.statusText = e.getStatusText();
            this.responseBody = e.getResponseBodyAsString();
            this.responseHeaders = e.getResponseHeaders();
        } else if (exception instanceof HttpClientErrorException) {
            HttpClientErrorException e = (HttpClientErrorException) exception;

            this.statusCode = e.getStatusCode();
            this.statusText = e.getStatusText();
            this.responseBody = e.getResponseBodyAsString();
            this.responseHeaders = e.getResponseHeaders();
        } else {
            this.statusText = exception.getMessage();
        }
    }

    public HttpStatus getStatusCode() {
        return statusCode;
    }

    public void setStatusCode(HttpStatus statusCode) {
        this.statusCode = statusCode;
    }

    public String getStatusText() {
        return statusText;
    }

    public void setStatusText(String statusText) {
        this.statusText = statusText;
    }

    public String getResponseBody() {
        return responseBody;
    }

    public void setResponseBody(String responseBody) {
        this.responseBody = responseBody;
    }

    public HttpHeaders getResponseHeaders() {
        return responseHeaders;
    }

    public void setResponseHeaders(HttpHeaders responseHeaders) {
        this.responseHeaders = responseHeaders;
    }
}

工具集

public class RestClient {

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param responseType 返回的数据类型
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, Class<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, null, null, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param headers      设置的头信息
     * @param responseType 返回的数据类型
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, HttpHeaders headers, Class<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, headers, null, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param body         要提交的数据
     * @param responseType 返回数据类型
     *                     返回bean时指定Class
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, Object body, Class<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, null, body, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param httpHeaders  请求头
     * @param body         要提交的数据
     * @param responseType 返回数据类型
     *                     返回bean时指定Class
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, HttpHeaders httpHeaders, Object body, Class<T> responseType, Object... uriVariables) throws RestClientException {
        try {
            HttpEntity<?> requestEntity = new HttpEntity(body, httpHeaders);
            requestEntity = convert(requestEntity);

            if (uriVariables.length == 1 && uriVariables[0] instanceof Map) {
                Map<String, ?> _uriVariables = (Map<String, ?>) uriVariables[0];
                return getClient().exchange(url, method, requestEntity, responseType, _uriVariables).getBody();
            }

            return getClient().exchange(url, method, requestEntity, responseType, uriVariables).getBody();
        } catch (Exception e) {
            throw new RestClientException(e);
        }
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param responseType 返回的数据类型，例：new ParameterizedTypeReference<List<Bean>>(){}
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, ParameterizedTypeReference<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, null, null, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param headers      设置的头信息
     * @param responseType 返回的数据类型，例：new ParameterizedTypeReference<List<Bean>>(){}
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, HttpHeaders headers, ParameterizedTypeReference<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, headers, null, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param body         要提交的数据
     * @param responseType 返回数据类型，例：new ParameterizedTypeReference<List<Bean>>(){}
     *                     返回bean时指定Class
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, Object body, ParameterizedTypeReference<T> responseType, Object... uriVariables) throws RestClientException {
        return exchange(url, method, null, body, responseType, uriVariables);
    }

    /**
     * 执行请求
     *
     * @param url          请求地址
     * @param method       请求方式
     * @param httpHeaders  请求头
     * @param body         要提交的数据
     * @param responseType 返回数据类型，例：new ParameterizedTypeReference<List<Bean>>(){}
     *                     返回bean时指定Class
     * @param uriVariables url自动匹配替换的参数，如url为api/{a}/{b},参数为["1","2"],则解析的url为api/1/2，使用Map参数时，遵循按key匹配
     * @return 结果对象
     * @throws RestClientException RestClient异常，包含状态码和非200的返回内容
     */
    public static <T> T exchange(String url, HttpMethod method, HttpHeaders httpHeaders, Object body, ParameterizedTypeReference<T> responseType, Object... uriVariables) throws RestClientException {
        try {
            HttpEntity<?> requestEntity = new HttpEntity(body, httpHeaders);
            requestEntity = convert(requestEntity);

            if (uriVariables.length == 1 && uriVariables[0] instanceof Map) {
                Map<String, ?> _uriVariables = (Map<String, ?>) uriVariables[0];
                return getClient().exchange(url, method, requestEntity, responseType, _uriVariables).getBody();
            }

            return getClient().exchange(url, method, requestEntity, responseType, uriVariables).getBody();
        } catch (Exception e) {
            throw new RestClientException(e);
        }
    }

    /**
     * 获得一个RestTemplate客户端
     *
     * @return
     */
    public static RestTemplate getClient() {
        return RestClientBuilder.build();
    }

    /**
     * 获取一个application/x-www-form-urlencoded头
     *
     * @return
     */
    public static HttpHeaders buildBasicFORMHeaders() {
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
        return headers;
    }

    /**
     * 获取一个application/json头
     *
     * @return
     */
    public static HttpHeaders buildBasicJSONHeaders() {
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);
        return headers;
    }

    /**
     * 获取一个text/html头
     *
     * @return
     */
    public static HttpHeaders buildBasicHTMLHeaders() {
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.TEXT_HTML);
        return headers;
    }

    /**
     * 构建一个json头
     *
     * @param arrays
     * @return
     */
    public static HttpHeaders buildJSONHeaders(Object... arrays) {
        if (arrays.length % 2 != 0) {
            throw new RuntimeException("arrays 长度 必须为偶数");
        }

        HttpHeaders headers = buildBasicJSONHeaders();

        for (int i = 0; i < arrays.length; i++) {
            headers.add(Lang.defaultEmptyStr(arrays[i]), Lang.defaultEmptyStr(arrays[++i]));
        }

        return headers;
    }

    /**
     * 对bean对象转表单模型做处理
     *
     * @param requestEntity
     * @return
     */
    private static HttpEntity<?> convert(HttpEntity<?> requestEntity) {
        Object body = requestEntity.getBody();
        HttpHeaders headers = requestEntity.getHeaders();

        if (body == null) {
            return requestEntity;
        }

        if (body instanceof Map) {
            MultiValueMap<String, String> multiValueMap = new LinkedMultiValueMap<>();
            Map<String, ?> _body = (Map<String, ?>) body;
            for (String key : _body.keySet()) {
                multiValueMap.add(key, MapUtils.getString(_body, key));
            }

            requestEntity = new HttpEntity<>(multiValueMap, headers);
        }

        if (headers == null || !MediaType.APPLICATION_FORM_URLENCODED.equals(headers.getContentType())) {
            return requestEntity;
        }

        if (body instanceof String) {
            return requestEntity;
        }

        if (body instanceof Collection) {
            return requestEntity;
        }

        if (body instanceof Map) {
            return requestEntity;
        }

        MultiValueMap<String, Object> formEntity = new LinkedMultiValueMap<>();

        Field[] fields = body.getClass().getDeclaredFields();
        for (int i = 0; i < fields.length; i++) {
            String name = fields[i].getName();
            String value = null;

            try {
                value = BeanUtils.getProperty(body, name);
            } catch (Exception e) {
                e.printStackTrace();
            }

            formEntity.add(name, value);
        }

        return new HttpEntity<>(formEntity, headers);
    }

    public final static Object[] EMPTY_URI_VARIABLES = new Object[]{};

    public final static HttpHeaders EMPTY_HEADERS = new HttpHeaders();

    public final static Map<String, ?> EMPTY_BODY = new HashMap<>(1);

    public final static HttpEntity EMPTY_ENTITY = new HttpEntity(EMPTY_HEADERS);

}

更多

RestTemplate API说明和使用参考

http://docs.spring.io/spring/docs/4.1.x/javadoc-api/org/springframework/web/client/RestTemplate.html

http://docs.spring.io/spring/docs/4.1.x/javadoc-api/org/springframework/http/client/SimpleClientHttpRequestFactory.html

http://docs.spring.io/spring/docs/4.1.x/javadoc-api/org/springframework/http/client/HttpComponentsClientHttpRequestFactory.html

HttpClient官方示例和参数配置说明

http://hc.apache.org/httpcomponents-client-4.4.x/examples.html

http://hc.apache.org/httpcomponents-client-4.4.x/tutorial/html/index.html

注意点

1.关于httpclient配置的defaultMaxPerRoute和maxTotal 
defaultMaxPerRoute：最大路由并发数，以主机为单位 
maxTotal：整个连接池的并发数

例如： 
defaultMaxPerRoute为10，maxTotal为100 
假设只会访问http://www.baidu.com和http://www.google.com 
那么能同时并发到客源的只能是10，房源也是10，整个连接永远不会到100

2.部分方法注意查看源码，默认构造里面会新增常用的数据转换器，spring对jackson比较情有独钟，在解析xml和json时，优先使用jackson

public RestTemplate() {
    this.messageConverters.add(new ByteArrayHttpMessageConverter());
    this.messageConverters.add(new StringHttpMessageConverter());
    this.messageConverters.add(new ResourceHttpMessageConverter());
    this.messageConverters.add(new SourceHttpMessageConverter<Source>());
    this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());

    if (romePresent) {
            this.messageConverters.add(new AtomFeedHttpMessageConverter());
            this.messageConverters.add(new RssChannelHttpMessageConverter());
    }
    if (jackson2XmlPresent) {
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
    }
    else if (jaxb2Present) {
            this.messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
    }
    if (jackson2Present) {
            this.messageConverters.add(new MappingJackson2HttpMessageConverter());
    }
    else if (gsonPresent) {
            this.messageConverters.add(new GsonHttpMessageConverter());
    }
}

/**
 * Create a new instance of the {@link RestTemplate} based on the given {@link ClientHttpRequestFactory}.
 * @param requestFactory HTTP request factory to use
 * @see org.springframework.http.client.SimpleClientHttpRequestFactory
 * @see org.springframework.http.client.HttpComponentsClientHttpRequestFactory
 */
public RestTemplate(ClientHttpRequestFactory requestFactory) {
    this();
    setRequestFactory(requestFactory);
}

再看添加转换器的方法外部添加转换器时，this.messageConverters.clear();会先清除已有的，需要注意

public RestTemplate(List<HttpMessageConverter<?>> messageConverters) {
    Assert.notEmpty(messageConverters, "'messageConverters' must not be empty");
    this.messageConverters.addAll(messageConverters);
}

/**
 * Set the message body converters to use.
 * <p>These converters are used to convert from and to HTTP requests and responses.
 */
public void setMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
    Assert.notEmpty(messageConverters, "'messageConverters' must not be empty");
    // Take getMessageConverters() List as-is when passed in here
    if (this.messageConverters != messageConverters) {
            this.messageConverters.clear();
            this.messageConverters.addAll(messageConverters);
    }
}