【发布时间】：2021-09-15 06:34:28
【问题描述】：
我正在开发一个经常使用 Jackson 的应用程序。为了了解readValue() 在某些极端情况下的工作原理，我设置了一个小实验。首先，一个故意为空的内部类：
static class Empty {
@Override
public String toString(){
return "Empty class!";
}
}
现在让我们设置一些打印输出：
public void testFromJsonStringBadInput() {
String[] emptyNullOrBadJsons = {"", "{}", "{", "}", "}{", "{{}", "{}}"};
Class<?>[] types = {Empty.class, Object.class};
for(String s: emptyNullOrBadJsons) {
for(Class<?> clazz : types) {
try {
System.out.println("new ObjectMapper().readValue(" + s + ", " + clazz.getSimpleName() + ")=" + new ObjectMapper().readValue(s, clazz));
} catch(Exception exc) {
System.out.println("Deserialization failure for Json string: " + s + " and class: " + clazz.getSimpleName() + ". Exception: " + exc.getClass().getSimpleName() +" and message: " + exc.getMessage());
}
}
}
}
因此，对于一堆极端情况的 JSON，我反序列化为 Empty 和 Object 的实例，并打印由 readValue() 生成的对象。这是这个小代码的输出：
Deserialization failure for Json string: and class: Empty. Exception: MismatchedInputException and message: No content to map due to end-of-input
at [Source: (String)""; line: 1, column: 0]
Deserialization failure for Json string: and class: Object. Exception: MismatchedInputException and message: No content to map due to end-of-input
at [Source: (String)""; line: 1, column: 0]
new ObjectMapper().readValue({}, Empty)=Empty class!
new ObjectMapper().readValue({}, Object)={}
Deserialization failure for Json string: { and class: Empty. Exception: JsonEOFException and message: Unexpected end-of-input: expected close marker for Object (start marker at [Source: (String)"{"; line: 1, column: 1])
at [Source: (String)"{"; line: 1, column: 2]
Deserialization failure for Json string: { and class: Object. Exception: JsonEOFException and message: Unexpected end-of-input: expected close marker for Object (start marker at [Source: (String)"{"; line: 1, column: 1])
at [Source: (String)"{"; line: 1, column: 2]
Deserialization failure for Json string: } and class: Empty. Exception: JsonParseException and message: Unexpected close marker '}': expected ']' (for root starting at [Source: (String)"}"; line: 1, column: 0])
at [Source: (String)"}"; line: 1, column: 2]
Deserialization failure for Json string: } and class: Object. Exception: JsonParseException and message: Unexpected close marker '}': expected ']' (for root starting at [Source: (String)"}"; line: 1, column: 0])
at [Source: (String)"}"; line: 1, column: 2]
Deserialization failure for Json string: }{ and class: Empty. Exception: JsonParseException and message: Unexpected close marker '}': expected ']' (for root starting at [Source: (String)"}{"; line: 1, column: 0])
at [Source: (String)"}{"; line: 1, column: 2]
Deserialization failure for Json string: }{ and class: Object. Exception: JsonParseException and message: Unexpected close marker '}': expected ']' (for root starting at [Source: (String)"}{"; line: 1, column: 0])
at [Source: (String)"}{"; line: 1, column: 2]
Deserialization failure for Json string: {{} and class: Empty. Exception: JsonParseException and message: Unexpected character ('{' (code 123)): was expecting double-quote to start field name
at [Source: (String)"{{}"; line: 1, column: 3]
Deserialization failure for Json string: {{} and class: Object. Exception: JsonParseException and message: Unexpected character ('{' (code 123)): was expecting double-quote to start field name
at [Source: (String)"{{}"; line: 1, column: 3]
new ObjectMapper().readValue({}}, Empty)=Empty class!
new ObjectMapper().readValue({}}, Object)={}
Process finished with exit code 0
我希望在最后一个字符串"{}}" 的所有情况下看到此输出除了。杰克逊反序列化器似乎很好地解析了这个 JSON，没有抛出异常，这对于我的自定义 Empty 类和 Object 都是成功的。怎么会？我本来希望readValue() 扔到这里。
如果它在任何方面很重要，这是我整个pom.xml 中唯一的杰克逊参考：
<dependency>
<groupId>com.fasterxml.jackson.dataformat</groupId>
<artifactId>jackson-dataformat-xml</artifactId>
</dependency>
【问题讨论】：
标签： json serialization jackson deserialization jsonserializer