查看“RestTemplate exchange”的源代码

https://www.bilibili.com/video/BV1AN411Z7mx?p=12

当请求的控制器返回值类型为 List<xx>、Set<xxx> 等带有泛型类型的类型时，就不能使用 getForObject、getForEntity、postForObject 等，因为它们都是只能设置返回值类型，而不能设置类型的泛型。

这时就需要用到 exchange 方法。

除此之外，如果需要设置请求头参数情况也需要使用 exchange 方法。

=== Application Service 中 ===

==== 新增 POJO People.java ====
<syntaxhighlight lang="java">
package io.github.jihch.pojo;

public class People {
    private int id;
    private String name;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public People() {
    }

    public People(int id, String name) {
        this.id = id;
        this.name = name;
    }
}
</syntaxhighlight>



==== DemoController.java 中新增方法 ====
<syntaxhighlight lang="java">
import io.github.jihch.pojo.People;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

@RestController
public class DemoController {

    @RequestMapping("/demo5")
    public People demo5() {
        return new People(1, "jihch");
    }

    @RequestMapping("/demo6")
    public List<People> demo6() {
        List<People> list = new ArrayList<>();
        list.add(new People(1, "jihch1"));
        list.add(new People(2, "jihch2"));
        return list;
    }

}
</syntaxhighlight>

=== Application Client 中的单元测试 ===

==== getForObject 获取简单非嵌套的单个对象 ====
<syntaxhighlight lang="java">
import io.github.jihch.pojo.People;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

import java.util.HashMap;
import java.util.Map;

@SpringBootTest
public class ClientApplicationTests {

    @Test
    void withObject() {
        RestTemplate restTemplate = new RestTemplate();
        People people = restTemplate.getForObject("http://localhost:8080/demo5", People.class);
        System.out.println(people);
        System.out.println(people.getId());
        System.out.println(people.getName());
    }

}
</syntaxhighlight><syntaxhighlight lang="console">
People{id=1, name='jihch'}
1
jihch
</syntaxhighlight>

==== getForObject 获取嵌套带泛型的对象就会遇到问题 ====
<syntaxhighlight lang="java">
import io.github.jihch.pojo.People;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

import java.util.HashMap;
import java.util.Map;

@SpringBootTest
public class ClientApplicationTests {

    /**
     * 接收返回值为 List<People> 这种带泛型的
     */
    @Test
    void withGenericity() {
        RestTemplate restTemplate = new RestTemplate();
        List<People> list = restTemplate.getForObject("http://localhost:8080/demo6", List.class);
        System.out.println(list);
        for (People people : list) {
            System.out.println(people);
        }
    }

}
</syntaxhighlight><syntaxhighlight lang="console">
[{id=1, name=jihch1}, {id=2, name=jihch2}]

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to io.github.jihch.pojo.People

	at io.github.jihch.ClientApplicationTests.withGenericity(ClientApplicationTests.java:111)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:675)
	at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:125)
	at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:132)
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:124)
	at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:74)
	at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
	at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:104)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:62)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:43)
	at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:35)
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
	at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:202)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:198)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:135)
	at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:69)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:135)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
	at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
	at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
	at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
	at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
	at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:229)
	at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:197)
	at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:211)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:191)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
	at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)
	at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)
	at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)
	at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
	at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
</syntaxhighlight>

==== exchange 获取带泛型的嵌套对象 ====
<syntaxhighlight lang="java">
import io.github.jihch.pojo.People;
import org.junit.jupiter.api.Test;
import org.junit.runners.Parameterized;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.core.ParameterizedTypeReference;
import org.springframework.http.HttpEntity;
import org.springframework.http.HttpMethod;
import org.springframework.http.ResponseEntity;
import org.springframework.web.client.RestTemplate;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

@SpringBootTest
public class ClientApplicationTests {

    @Test
    void exchange() {
        RestTemplate restTemplate = new RestTemplate();
        // HttpEntity 请求体数据
        // 构造方法参数是请求体内容
        HttpEntity<String> httpEntity = new HttpEntity<>("");
        // 泛型为响应体的类型
        ParameterizedTypeReference<List<People>> parameterizedTypeReference =
                new ParameterizedTypeReference<List<People>>() {
        };
        ResponseEntity<List<People>> result = restTemplate.exchange("http://localhost:8080/demo6", HttpMethod.GET,
                null, parameterizedTypeReference);
        List<People> list = result.getBody();
        for (People people : list) {
            System.out.println(people);
        }
    }

}
</syntaxhighlight><syntaxhighlight lang="console">
People{id=1, name='jihch1'}
People{id=2, name='jihch2'}
</syntaxhighlight>