为什么此代码在包含在通用函数中时不起作用？

Question

我使用此代码执行 HTTP POST 请求并反序列化返回值：

ParameterizedTypeReference<MyClass> typeRef = new ParameterizedTypeReference<>() {};
HttpEntity<Object> requestEntity = new HttpEntity<>("some text");
ResponseEntity<MyClass> result = restTemplate.exchange("/test", HttpMethod.POST, requestEntity, typeRef);
MyClass returnValue = result.getBody();

为了更易于使用，我尝试将代码包装在一个函数中，如下所示：

public <T> T post(Object content, Class<T> returnType, String url){
    ParameterizedTypeReference<T> typeRef = new ParameterizedTypeReference<>() {};
    HttpEntity<Object> requestEntity = new HttpEntity<>(content);
    ResponseEntity<T> response = restTemplate.exchange(url, HttpMethod.POST, requestEntity, typeRef);
    return response.getBody();
}

但是，代码在放入函数时停止运行。它抛出java.lang.ClassCastException: java.base/java.util.LinkedHashMap cannot be cast to client.rest.MyClass. 似乎某些类型信息在此过程中的某处丢失了。

这是2个测试用例形式的完整代码：

package client.rest;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.junit.BeforeClass;
import org.junit.Test;
import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.*;
import org.springframework.test.web.client.MockRestServiceServer;
import org.springframework.web.client.RestTemplate;
import org.springframework.web.client.support.RestGatewaySupport;

import static org.springframework.test.web.client.ExpectedCount.times;
import static org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo;
import static org.springframework.test.web.client.response.MockRestResponseCreators.withSuccess;

class MyClass {
    public int getInt(){
        return 1;
    }

    public void setInt(int i){}
}

public class TEMP {

    public static RestTemplate restTemplate = new RestTemplate();
    public static MockRestServiceServer mockServer;

    @BeforeClass
    public static void beforeClass() throws JsonProcessingException {
        MyClass value = new MyClass();

        // set up a mock server
        RestGatewaySupport gateway = new RestGatewaySupport();
        gateway.setRestTemplate(restTemplate);
        mockServer = MockRestServiceServer.bindTo(gateway).build();

        ObjectMapper objectmapper = new ObjectMapper();
        String payload = objectmapper.writeValueAsString(value);

        mockServer.expect(times(2), requestTo("/test"))
            .andRespond(withSuccess(payload, MediaType.APPLICATION_JSON));
    }

    @Test
    public void without_function() {
        ParameterizedTypeReference<MyClass> typeRef = new ParameterizedTypeReference<>() {};
        HttpEntity<Object> requestEntity = new HttpEntity<>("some text");
        ResponseEntity<MyClass> result = restTemplate.exchange("/test", HttpMethod.POST, requestEntity, typeRef);
        MyClass returnValue = result.getBody();
    }

    public <T> T post(Object content, Class<T> returnType, String url){
        ParameterizedTypeReference<T> typeRef = new ParameterizedTypeReference<>() {};
        HttpEntity<Object> requestEntity = new HttpEntity<>(content);
        ResponseEntity<T> response = restTemplate.exchange(url, HttpMethod.POST, requestEntity, typeRef);
        return response.getBody();
    }

    @Test
    public void with_function() {
        MyClass returnValue = post("some text", MyClass.class, "/test");
    }
}

我的问题是双重的：

1. 为什么该功能不起作用？
2. 我怎样才能让它工作？

Answer 1

回答 1。

ParameterizedTypeReference<X> typeRef = new ParameterizedTypeReference<X>() {};

感谢最后的{}jackson 能够X使用反射找出运行时的内容，但是X在编译时解决，所以如果你有MyClass或者T这正是它在运行时会得到的；它将无法弄清楚T在运行时分配给什么。

出于同样的原因，如果您继续使用无函数选项，但{}在最后删除了它，它将编译但会导致相同的错误。

回答2。

Class<T> returnType顺便说一句，您可以ParameterizedTypeReference<T> typeRef直接通过，而不是您从不引用 btw 。调用 post 的代码需要T在编译时确定：

@Test
    public void with_function() {
        ParameterizedTypeReference<MyClass> typeRef = new ParameterizedTypeReference<>() {}; 
        MyClass returnValue = post("some text", typeRef, "/test");
    }
}

但是，我认为您应该考虑不依赖{}可能有问题的技巧的替代方案。

您是否尝试过ParameterizedTypeReference的forType？

public <T> T post(Object content, Class<T> returnType, String url){
        ParameterizedTypeReference<T> typeRef = ParameterizedTypeReference.forType(returnType);
        HttpEntity<Object> requestEntity = new HttpEntity<>(content);
        ResponseEntity<T> response = restTemplate.exchange(url, HttpMethod.POST, requestEntity, typeRef);
        return response.getBody();
}

在任何情况下，这都适用于作为返回类型传递时T喜欢的非泛型赋值；它不起作用，因为它等同于 return 。我猜在这些情况下，您需要构造并传递一个与更复杂的类型表达式相对应的不同实例。MyClassMyClass.classArrayList<MyClass> list; list.getClass()ArrayList.classType