[英]How to map a simple Kotlin data class to/from json with Jackson and spring-boot
I cannot read a simple kotlin data class from a json string. 我无法从json字符串中读取简单的kotlin数据类。 Writing to json works fine, but when reading from json:
写json工作正常,但从json读取时:
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `my.package.Pojo` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
I am using spring-boot 2.1.5: 我使用的是spring-boot 2.1.5:
A simple junit test to recreate the exception: 一个简单的junit测试来重新创建异常:
package my.package;
import static org.assertj.core.api.Assertions.assertThat;
import static org.junit.jupiter.api.Assertions.assertAll;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.IOException;
import org.junit.jupiter.api.DisplayName;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
@SpringBootTest(classes = MyApp.class)
public class DtoMapperTest {
@Autowired
private ObjectMapper objectMapper;
@Test
@DisplayName("should map to/from json")
void shouldMapJson() throws IOException {
String json = objectMapper.writeValueAsString(new Pojo("1"));
assertThat(json).contains("\"id\":\"1\"");
Pojo fromJson = objectMapper.readValue(json, Pojo.class);
assertThat(fromJson).isEqualTo(new Pojo("1"));
}
}
My kotlin Pojo: 我的kotlin Pojo:
data class Pojo(
var id: String,
var anObject: AnotherPojo?
) {
constructor(ident: String) : this(ident, null)
// some business logic...
}
The exception: 例外:
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `my.package.Pojo` (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)
at [Source: (String)"{"id":"1","anObject":null}"; line: 1, column: 2]
at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1343)
at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1032)
at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1297)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:326)
at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3004)
at no.nav.bidrag.dokument.DtoMapperTest.shouldMapJson(DtoMapperTest.java:28)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:567)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:532)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:115)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:171)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:167)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:114)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:59)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$4(NodeTestTask.java:108)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:98)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:74)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$4(NodeTestTask.java:112)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:98)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:74)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$4(NodeTestTask.java:112)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:72)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:98)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:74)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:220)
at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$6(DefaultLauncher.java:188)
at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:202)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:181)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:128)
at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:69)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
this is a newbie error in the Kotlin department... 这是Kotlin部门的一个新手错误......
all my other pojos have nullable fields and do not need a no-arg constructor for deserialisation... The id of my pojo is a String (not-nullable in Kotlin) which is to say that you need a no-arg constructor which set the field to an empty string... 我所有的其他pojos都有可空的字段,并且不需要用于反序列化的no-arg构造函数...我的pojo的id是一个String(在Kotlin中不可为空),也就是说你需要一个无参数的构造函数字段为空字符串...
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