RestTemplate请求的服务实例返回List类型，用数组接收

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本文介绍使用Spring的RestTemplate从服务端获取List类型的对象时遇到的问题及解决方案。通过修改服务端返回类型为数组并转换为List的方式成功解决问题。

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使用RestTemplate请求的服务实例返回List类型，若使用List类型解析，会出现问题

例有user类如下

 public class User {
    private Long id;
    private String username;
    private String name;
    private Short age;
    private BigDecimal balance;
    public Long getId() {
        return this.id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getUsername() {
        return this.username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getName() {
        return this.name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Short getAge() {
        return this.age;
    }
    public void setAge(Short age) {
        this.age = age;
    }
    public BigDecimal getBalance() {
        return this.balance;
    }
    public void setBalance(BigDecimal balance) {
        this.balance = balance;
    }
}

服务提供端代码如下：

@GetMapping("list-all")
public List<User> listAll() {
    ArrayList<User> list = Lists.newArrayList();
    User user = new User(1L, "zhangsan");
    User user2 = new User(2L, "zhangsan");
    User user3 = new User(3L, "zhangsan");
    list.add(user);
    list.add(user2);
    list.add(user3);
    return list;
}

服务消费端代码如下：

@GetMapping("/test-list-all")
public List<User> listAll() {
    List<User> userList = restTemplate.getForObject("http://microservice-provider-user/list-all", List.class);
    for (User user : userList) {
        System.out.println(user.getId());
    }
    return  userList;
}

在页面请求：服务消费方URL/test-list-all，会出现如下报错：

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.itmuch.cloud.entity.User
	at com.itmuch.cloud.controller.MovieController.listAll(MovieController.java:45) ~[classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:220) ~[spring-web-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:134) ~[spring-web-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:116) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:622) ~[tomcat-embed-core-8.5.6.jar:8.5.6]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.4.RELEASE.jar:4.3.4.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.5.6.jar:8.5.6]

把服务端代码改为数组接收，在将数组转化为List。

@GetMapping("/test2-list-all")
public List<User> listAll2() {
    User[] users = restTemplate.getForObject("http://microservice-provider-user/list-all", User[].class);
    List<User> userList = Arrays.asList(users);
    for (User user : userList) {
        System.out.println(user.getId());
    }
    return  userList;
}

如上请求，可以正常获取服务返回的List类型数据。