为什么这段代码包装在泛型函数中不起作用？

Question

我使用以下代码执行HTTP POST请求并反序列化返回的值：

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();

为了让它更容易使用，我尝试将代码封装在一个函数中，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();
}

然而，当代码被放入函数中时，它就会停止工作。它抛出java.lang.ClassCastException:java.base/java.util.LinkedHashMap不能强制转换为client.rest.MyClass。似乎有些类型信息在途中丢失了。

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");
    }
}

null

Answer 1

1的答案。

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

感谢最后的{}jackson能够使用反射找出X在运行时是什么，但是X是在编译时解析的，所以如果您有MyClass或T，这正是它在运行时将得到的；它将无法确定t在运行时被分配给什么。

出于同样的原因，如果您继续使用function-less选项，但在最后删除{}，它将编译，但将导致相同的错误。

@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{}技巧的替代方案，这可能会有问题。

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，这都将处理对t的非泛型赋值，比如myclass，就像将myclass.class作为返回类型传递时一样；它不能用于ArrayList 列表；List.getClass() ，因为它相当于返回ArrayList.class。我想在这些情况下，您需要构造并传递一个不同的type实例，该实例对应于更复杂的类型表达式。