问题
I tried using @NotNull annotation but it did not work. If any field is missing in the JsonString, it gives null(String) or zero(int). But what I want is if any field is missing in the JsonStr defined in the class should throw an exception.
Add: My PojoClass may have object reference or multilevel object reference. I am using Gson for the conversion of the String to obj.
for more clarification, I have added my code below:
JsonStr:
{
"name":"John",
"id":1,
"roll":100,
"c":{
"city":"Dhaka",
"school":"B. govt. School"
}
}
Code:
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;
}
}
ClassB:
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;
}
}
MainClass:
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");
}
For the field roll, I used @NotNull to throw an exception if that field is not present in the JsonStr but it gives 0 value without throwing any exception.
How can I implement that?
Please don't say this is duplicate, because I have seen these questions:
GSON throwing null pointer exception when a field is missing in json [duplicate]
What is a NullPointerException, and how do I fix it?
回答1:
@NotNull is not a part of Gson, and it cannot handle it by default (unless you add support yourself). Also, nullability-check annotations should not be applied to primitives fields -- generally speaking it makes not much sense. Also, the default int fields values are 0, and Gson does not check if a particular field was read from a JSON document. However, you can easily implement it if you:
- slightly change your mapping by changing the primitive
intto its nullable wrapperInteger; - implement a custom post-processing validation type adapter.
For example,
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;
}
The next step is just creating your own type adapter to check the nullable fields.
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;
}
}
Test (uses some Google Guava and custom resource readers):
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());
}
}
}
}
Output:
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
Similarly, you can create your another TypeAdapter to check all fields automatically not even needing @NotNull, but this needs a more complicated implementation.
回答2:
Have you try using Wrapper class instead primitive?
use this:
@NotNull
private Integer roll;
来源：https://stackoverflow.com/questions/44362030/how-to-throw-exception-while-converting-from-jsonstr-to-pojoclassobj-if-any-key