我尝试使用@NotNull注释，但它不起作用。如果 JsonString 中缺少任何字段，则会给出 null(String) 或 zero(int)。但我想要的是，如果类中定义的 JsonStr 中缺少任何字段，则应抛出异常。
添加:我的PojoClass可能有对象引用或多级对象引用。我正在使用Gson将String转换为obj。
为了获得更多说明，我在下面添加了代码:
JsonStr:
{
"name":"John",
"id":1,
"roll":100,
"c":{
"city":"Dhaka",
"school":"B. govt. School"
}
}
代码:
public class C {
private String city;
private String school;
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public String getSchool() {
return school;
}
public void setSchool(String school) {
this.school = school;
}
}
B类:
public class B {
private String name;
private int id;
@NotNull
private int roll;
private C c;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getRoll() {
return roll;
}
public void setRoll(int roll) {
this.roll = roll;
}
public C getC() {
return c;
}
public void setC(C c) {
this.c = c;
}
}
主类:
try {
B obj = new B();
String str = "{\"name\":\"John\",\"id\":1,\"c\":{\"city\":\"dhaka\",\"school\":\"school\"}}";
obj = gson.fromJson(str, B.class);
} catch (RuntimeException e) {
System.out.println("exception Message");
}
对于字段滚动，如果 JsonStr 中不存在该字段，我使用 @NotNull 抛出异常，但它给出 0 值而不抛出任何异常。
我该如何实现？
请不要说这是重复的，因为我已经看到了这些问题:
最佳答案
@NotNull 不是 Gson 的一部分，默认情况下它无法处理它(除非您自己添加支持)。此外，可空性检查注释不应该应用于基元字段——一般来说，它没有多大意义。此外，默认的 int 字段值为 0，并且 Gson 不会检查是否从 JSON 文档读取特定字段。但是，如果您满足以下条件，则可以轻松实现它:
- 通过将基元
int更改为其可为 null 的包装器Integer来稍微更改映射； - 实现自定义后处理验证类型适配器。
例如，
final class Person {
final String name = null;
final int id = Integer.valueOf(0); // final primitive fields are inlined by javac
@NotNull
final Integer roll = null;
@SerializedName("c")
final Location location = null;
}
final class Location {
final String city = null;
final String school = null;
}
下一步就是创建您自己的类型适配器来检查可为空字段。
final class NotNullTypeAdapterFactory
implements TypeAdapterFactory {
// The type adapter factory holds no state, so no need to instantiate it multiple times
private static final TypeAdapterFactory notNullTypeAdapterFactory = new NotNullTypeAdapterFactory();
private NotNullTypeAdapterFactory() {
}
static TypeAdapterFactory getNotNullTypeAdapterFactory() {
return notNullTypeAdapterFactory;
}
@Override
public <T> TypeAdapter<T> create(final Gson gson, final TypeToken<T> typeToken) {
final Collection<Field> notNullFields = getNotNullFields(typeToken.getRawType());
// If no @NotNull fields found, then just tell Gson to pick the next best type adapter
if ( notNullFields.isEmpty() ) {
return null;
}
// If there's at least one @NotNull field, get the original type adapter
final TypeAdapter<T> delegateTypeAdapter = gson.getDelegateAdapter(this, typeToken);
return new TypeAdapter<T>() {
@Override
public void write(final JsonWriter out, final T value)
throws IOException {
delegateTypeAdapter.write(out, value);
}
@Override
public T read(final JsonReader in)
throws IOException {
try {
// Read the value ...
final T value = delegateTypeAdapter.read(in);
// ... and make some post-processing
for ( final Field f : notNullFields ) {
if ( f.get(value) == null ) {
throw new MalformedJsonException(f + " has no value");
}
}
return value;
} catch ( final IllegalAccessException ex ) {
throw new IOException(ex);
}
}
};
}
private static Collection<Field> getNotNullFields(final Class<?> clazz) {
// Primitive types and java.lang.Object do not have @NotNull
if ( clazz.isPrimitive() || clazz == Object.class ) {
return emptyList();
}
// Scan the whole hierarchy from the bottom subclass to the top superclass (except java.lang.Object we mentioned above)
final Collection<Field> notNullFields = new ArrayList<>();
for ( Class<?> c = clazz; c != Object.class; c = c.getSuperclass() ) {
for ( final Field f : c.getDeclaredFields() ) {
if ( f.isAnnotationPresent(NotNull.class) ) {
// Don't forget to make private fields accessible
f.setAccessible(true);
notNullFields.add(f);
}
}
}
return notNullFields;
}
}
测试(使用一些 Google Guava 和自定义资源读取器):
private static final Gson gson = new GsonBuilder()
.registerTypeAdapterFactory(getNotNullTypeAdapterFactory())
.create();
public static void main(final String... args)
throws IOException {
for ( final String resourceName : ImmutableList.of("file-with-roll.json", "file-without-roll.json") ) {
System.out.println("Deserializing " + resourceName);
try ( final JsonReader jsonReader = getPackageResourceJsonReader(Q44362030.class, resourceName) ) {
try {
final Person person = gson.fromJson(jsonReader, Person.class);
System.out.println(person.name + " " + person.roll);
} catch ( final Exception ex ) {
System.out.println("FAILED! " + ex.getMessage());
}
}
}
}
输出:
Deserializing file-with-roll.json
John 100
Deserializing file-without-roll.json
FAILED! com.google.gson.stream.MalformedJsonException: final java.lang.Integer q44362030.Person.roll has no value
类似地，您可以创建另一个 TypeAdapter 来自动检查所有字段，甚至不需要 @NotNull，但这需要更复杂的实现。
关于java - 如果 JsonStr 中缺少任何键/值，如何在从 JsonStr 转换为 PojoClassObj 时抛出异常？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/44362030/