RestTemplate入门

本篇主要讲解RestTemplate的基本使用，它是Spring提供的用来访问Rest服务的客户端，RestTmplate提供了很多便捷的方法，可以大大提供开发效率，本篇只涉及基本使用，内部原理后续再展开

1.RestTemplate简述

RestTemplate是Spring提供的用于发送HTTP请求的客户端工具，它遵循Restful原则，RestTemplate默认依赖JDK的Http连接工具HttpUrlConnection,你也可以替换不同的源，比如OkHttp、Apache HttpComponents 等等。。

2.HttpMessageConverter

在说RestTemplate之前,先介绍HttpMessageConverter，RestTemplate默认使用的转化HttpMessageConverter去将Http消息转换成POJO 或者 POJO转化成Http消息。在创建RestTemplate的时候会默认添加一组HttpMessageConveter的实现。

HttpMessageConverter源码：

public interface HttpMessageConverter<T> {
    //指示此转换器是否可以读取给定的类。
	boolean canRead(Class<?> clazz, [@Nullable](https://my.oschina.net/u/2896689) MediaType mediaType);

    //指示此转换器是否可以写给定的类。
	boolean canWrite(Class<?> clazz, [@Nullable](https://my.oschina.net/u/2896689) MediaType mediaType);

    //返回List<MediaType>
	List<MediaType> getSupportedMediaTypes();

    //读取一个inputMessage
	T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
        throws IOException, HttpMessageNotReadableException;

    //往output message写一个Object
	void write(T t, [@Nullable](https://my.oschina.net/u/2896689) MediaType contentType, HttpOutputMessage outputMessage)
        throws IOException, HttpMessageNotWritableException;

}

RestTemplate中如何添加HttpMessageConverter的

构造器源码：

static {
	ClassLoader classLoader = RestTemplate.class.getClassLoader();
	romePresent = ClassUtils.isPresent("com.rometools.rome.feed.WireFeed", classLoader);
	jaxb2Present = ClassUtils.isPresent("javax.xml.bind.Binder", classLoader);
	jackson2Present =
			ClassUtils.isPresent("com.fasterxml.jackson.databind.ObjectMapper", classLoader) &&
					ClassUtils.isPresent("com.fasterxml.jackson.core.JsonGenerator", classLoader);
	jackson2XmlPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.xml.XmlMapper", classLoader);
	jackson2SmilePresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.smile.SmileFactory", classLoader);
	jackson2CborPresent = ClassUtils.isPresent("com.fasterxml.jackson.dataformat.cbor.CBORFactory", classLoader);
	gsonPresent = ClassUtils.isPresent("com.google.gson.Gson", classLoader);
	jsonbPresent = ClassUtils.isPresent("javax.json.bind.Jsonb", classLoader);
}


public RestTemplate() {
	this.messageConverters.add(new ByteArrayHttpMessageConverter());
	this.messageConverters.add(new StringHttpMessageConverter());
	this.messageConverters.add(new ResourceHttpMessageConverter(false));
	try {
		this.messageConverters.add(new SourceHttpMessageConverter<>());
	}
	catch (Error err) {
		// Ignore when no TransformerFactory implementation is available
	}
	this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());

	if (romePresent) {
		this.messageConverters.add(new AtomFeedHttpMessageConverter());
		this.messageConverters.add(new RssChannelHttpMessageConverter());
	}

	if (jackson2XmlPresent) {
		this.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());
	}
	else if (jsonbPresent) {
		this.messageConverters.add(new JsonbHttpMessageConverter());
	}

	if (jackson2SmilePresent) {
		this.messageConverters.add(new MappingJackson2SmileHttpMessageConverter());
	}
	if (jackson2CborPresent) {
		this.messageConverters.add(new MappingJackson2CborHttpMessageConverter());
	}

	this.uriTemplateHandler = initUriTemplateHandler();
}

从RestTemplate的构造器源码中我们可以看到构造方法中根据ClassUtils.isPresent判断当前项目是否存在某些jar包向RestTemplate中添加不同的 HttpMessageConverter实现，Spring默认会添加 StringHttpMessageConverter ， MappingJackson2XmlHttpMessageConverter等等

3.getForObject()

首先来看Get请求 RestTemplate提供了2种方法其中一种就是 getForObject()方法

public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables){}
public <T> T getForObject(String url, Class<T> responseType, Map<String, ?> uriVariables)
public <T> T getForObject(URI url, Class<T> responseType)

3.1 不带参数的Get请求

客户端
 String url = "http://localhost:9999/testGetMethod";
 String str = restTemplate.getForObject(url , String.class);、

服务端
@RequestMapping("/testGetMethod")
public String testGetMethod(){
    return "hello";
}

3.2 带参数的Get请求（按{}顺序绑定参数）

客户端
 String url = "http://localhost:9999/testGetMethod/{talk}/{name}"; 
	//或者testGetMethod/{1}/{2} {里面的参数无所谓，只是根据下面的Object... uriVariables 逐一绑定到上面{} 中去的}

 String str = restTemplate.getForObject(url , String.class,"hello" , "johnny");

服务端
  @RequestMapping("/testGetMethod/{talk}/{name}")
public String testGetMethod(@PathVariable("talk") String talk ,
                            @PathVariable("name") String name){
    log.info("【{} : {}】" , talk , name);
    return "hello";
}

说明{} http://localhost:9999/testGetMethod/{talk}/{name} 里面的参数名称无所谓，只是根据下面的Object... uriVariables 逐一绑上去的，具体绑定源码请看 DefaultUriBuilderFactory的fromUriString方法内部是根据正则表达式进行绑定的，最终将返回 URI，URI的string属性就是绑定后要访问的路径

//不早了先写这么多吧明天把下面的补齐，先去睡觉了！！！ //继续补齐。。

3.3 带参数的Get请求（按Map的key绑定{}参数）

 String url = "http://localhost:9999/testGetMethod/{talk}/{name}";
 Map<String,String> map = new HashMap<>(); 
 map.put("talk" , "hello"); 
 map.put("name" , "johnny"); 
 String str = restTemplate.getForObject(url , String.class,map);

可以看出来是通过Map绑定参数的，这个和上面不一样的是 map的key 一定要和 url中 {} 中的保持一致，如果不一致会报错

 String url = "http://localhost:9999/testGetMethod/{talk}/{name}"; 
 map.put("talk2" , "hello");

源码

4.getForEntity()

RestTemplate 提供的Get请求的第二种方法，基本和getForObject一致，就是返回类型不同，它是在getForObject返回的基础上再包装了一层 ResponseEntity 用于包含Http请求的全部信息

public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... uriVariables){}
public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Map<String, ?> uriVariables){}
public <T> ResponseEntity<T> getForEntity(URI url, Class<T> responseType){}

4.1 不带参数的Get请求

    String url = "http://localhost:9999/testGetMethodForEntity/";
    ResponseEntity<Student> responseEntity = restTemplate.getForEntity(url , Student.class);
    responseEntity.getStatusCode();
    responseEntity.getHeaders();
    responseEntity.getBody();
    log.info("【responseEntity : {}】" , responseEntity);

4.2 带参数的Get请求（按{}顺序绑定参数）

    String url = "http://localhost:9999/testGetMethodForEntity/{name}/{age}";
    ResponseEntity<Student> responseEntity = restTemplate.getForEntity(url , Student.class , "johnny" , "23");
    log.info("【responseEntity : {}】" , responseEntity);

4.3 带参数的Get请求（按Ma的keyp绑定{}参数）

    String url = "http://localhost:9999/testGetMethodForEntity/{name}/{age}";
    Map<String,String> map = new HashMap<>();
    map.put("name","johnny");
    map.put("age" , "23");
    ResponseEntity<Student> responseEntity = restTemplate.getForEntity(url , Student.class ,"map");
    log.info("【responseEntity : {}】" , responseEntity);

但是,通常情况下我们并不想要Http请求的全部信息,只需要相应体即可.对于这种情况,RestTemplate提供了 getForObject() 方法用来只获取响应体信息. getForObject 和 getForEntity 用法几乎相同,指示返回值返回的是响应体,省去了我们再去 getBody() .

5.postForObject()

public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Object... uriVariables)
        throws RestClientException {}
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables)
        throws RestClientException {}
public <T> T postForObject(URI url, @Nullable Object request, Class<T> responseType) throws RestClientException {}

5.1 post请求提交参数 Student 对象

客户端代码：

    String url = "http://localhost:9999/testPostMethodForObject";
    Student student = new Student();
    student.setName("johnny");
    student.setAge("23");

    String msg = restTemplate.postForObject(url , student , String.class);

服务端代码：
     @RequestMapping("/testPostMethodForObject")
    public String testGetMethodForEntity(@RequestBody Student student){
     log.info("【studeng: {}】"  , student);
     return "success";
   }

6.postForEntity()

public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Object... uriVariables)
        throws RestClientException {}
public <T> T postForObject(String url, @Nullable Object request, Class<T> responseType, Map<String, ?> uriVariables)
        throws RestClientException {}
public <T> T postForObject(URI url, @Nullable Object request, Class<T> responseType) throws RestClientException {}

6.1 post请求提交参数 Student 对象

@RequestMapping("/testPostMethodForEntity")
public void testPostMethodForEntity(){

    String url = "http://localhost:9999/testPostMethodForEntity";
    Student student = new Student();
    student.setName("johnny");
    student.setAge("23");

    ResponseEntity<String> responseEntity = restTemplate.postForEntity(url , student , String.class);
    log.info(responseEntity.getBody());
}