com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.lang.String` out of START_ARRAY token
 at [Source: (String)"{"a":"hnsdklnd","b":[]}"; line: 1, column: 21] (through reference chain: hzw.test.Entity["b"])
	at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)
	at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1342)
	at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1138)
	at com.fasterxml.jackson.databind.deser.std.StdDeserializer._deserializeFromArray(StdDeserializer.java:674)
	at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:40)
	at com.fasterxml.jackson.databind.deser.std.StringDeserializer.deserialize(StringDeserializer.java:10)
	at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:127)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.vanillaDeserialize(BeanDeserializer.java:287)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:151)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2992)
	at hzw.test.testFrequentlyMethod.comFasterxmlJacksonDatabindObjectMapperReadValue(testFrequentlyMethod.java:285)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
	at java.lang.reflect.Method.invoke(Unknown Source)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

@Test
	public void testComFasterxmlJacksonDatabindObjectMapperReadValue() {

		String json = "{\"a\":\"hnsdklnd\",\"b\":[]}";
		ObjectMapper objectMapper = new ObjectMapper();
		Entity readValue;
		try {
			readValue = objectMapper.readValue(json, Entity.class);
			System.out.println(readValue.toString());
		} catch (JsonParseException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (JsonMappingException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}

	}

package hzw.test;

public class Entity {
	String a;
	String b;

	public String getA() {
		return a;
	}

	public void setA(String a) {
		this.a = a;
	}

	public String getB() {
		return b;
	}

	public void setB(String b) {
		this.b = b;
	}

	@Override
	public String toString() {
		return "Entity [a=" + a + ", b=" + b + "]";
	}

}

上图三段代码分别为错误信息，调用信息，实体类。

补充：com.fasterxml.jackson.databind.ObjectMapper.readValue(String, Class<Entity>)方法作用为：Method to deserialize JSON content from given JSON content String（方法从给定的JSON内容字符串反序列化JSON内容（百度翻译），意思就是说从json格式转化成你设置的实体类）

前提条件：自定义Json格式字符串变量json为（a=hnsdklnd, b=[]）中b为空值。

错误信息：Cannot deserialize instance of `java.lang.String` out of START_ARRAY token（不能反序列化START_ARRAY令牌中的String类型实例）（百度翻译）

错误原因：在实体类中定义两个String类型变量a,b。在自定义Json格式字符串变量json为（a=hnsdklnd, b=[]），很明显json格式中b的数据格式不匹配（为数组类型）

解决方案：1.改变json格式字符串中b的格式为字符串（a=hnsdklnd, b=null），实体类型不变。

                  2.改变实体类中变量b的类型为字符串数组类型（String[]），json格式字符串不变。