使用GSON和TypeAdapter将BSON（mongoDB）读入POJO [英] Read BSON (mongoDB) into POJO using GSON and TypeAdapter
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问题描述
我正在寻找一种使用GSON将MongoDB文档读入POJO的方法。它可以很好地工作，直到遇到诸如date和longs之类的东西。
我想为Gson编写一个自定义适配器，它将转换任何长度为BSON的编码。阅读此发布我已创建我自己的适配器：
public class BsonLongTypeAdapter extends TypeAdapter< Long>
{
@Override
public void write（JsonWriter out，Long value）throws IOException
{
out.beginObject（）
.name（$ numberLong）
.value（value.toString（））
.endObject（）;
}
@Override
public Long read（JsonReader in）throws IOException
{
in.beginObject（）;
断言$ numberLong.equals（in.nextName（））;
Long value = in.nextLong（）;
in.endObject（）;
返回值;
$ b $ p
$ b我已经定义了以下测试来检查这个工作是否正常： @Test
public void canWriteCorrectJSON（）{
Gson gson = new GsonBuilder（）。registerTypeAdapter （Long.class，new BsonLongTypeAdapter（））。create（）;
MyTestObject obj = new MyTestObject（1458569479431L）;
字符串gsonString = gson.toJson（obj）;
assertEquals（{\timestamp \：{\$ numberLong \：\1458569479431\}}，gsonString）;
$ b @Test
public void canReadFromJSON（）{
Gson gson = new GsonBuilder（）。registerTypeAdapter（Long.class，new BsonLongTypeAdapter（））。create （）;
MyTestObject actualTaskObject = gson.fromJson（{\timestamp \：{\$ numberLong \：\1458569479431 \}}，MyTestObject.class）;
MyTestObject taskObject = new MyTestObject（1458569479431L）;
assertEquals（taskObject.getTimestamp（），actualTaskObject.getTimestamp（））;
}
私有静态类MyTestObject
{
long timestamp;
public MyTestObject（long ts）
{
timestamp = ts;
}
public long getTimestamp（）
{
return timestamp;
}
public void setTimestamp（long timestamp）
{
this.timestamp = timestamp;
$ / code>
第一个（写入）测试工作得很好，但阅读测试失败：
com.google.gson.JsonSyntaxException：java.lang.IllegalStateException：预计会长，但是BEGIN_OBJECT在第1行第15列路径$ .timestamp
因为从我的适配器读取函数永远不会被调用。我推测这可能是因为我想映射到MyTestObject而不是Long，但我不想为所有包含long的类编写适配器。
是否可以编写一个适用于GSON的适配器来转换所有发送给它的BSON多长时间？解决方案
我使用CustomizedTypeAdapterFactory 。 看到这个问题 b
$ b
基本上先写一个自定义的适配器：
public abstract class CustomizedTypeAdapterFactory< c取代;
实现TypeAdapterFactory
{
private final Class< C> customizedClass;
public CustomizedTypeAdapterFactory（Class< C> customizedClass）{
this.customizedClass = customizedClass;
}
@SuppressWarnings（unchecked）//我们使用运行时检查来保证'C'和'T'相等
public final< T> TypeAdapter< T> create（Gson gson，TypeToken< T> type）{
return type.getRawType（）== customizedClass
？ （TypeAdapter< T>）customizeMyClassAdapter（gson，（TypeToken< C>）类型）
：null;
}
私人TypeAdapter< C> customizeMyClassAdapter（Gson gson，TypeToken< C>类型）{
final TypeAdapter< C> delegate = gson.getDelegateAdapter（this，type）;
final TypeAdapter< JsonElement> elementAdapter = gson.getAdapter（JsonElement.class）;
return new TypeAdapter< C>（）{
@Override public void write（JsonWriter out，C value）throws IOException
{
JsonElement tree = delegate.toJsonTree（value）;
beforeWrite（value，tree）;
elementAdapter.write（out，tree）;
@Override public C read（JsonReader in）throws IOException {
JsonElement tree = elementAdapter.read（in）;
afterRead（tree）;
返回delegate.fromJsonTree（tree）;
}
};
}
/ **
*在写入
*输出的JSON流之前，用{@code toSerialize}覆盖它。
* /
protected void beforeWrite（C source，JsonElement toSerialize）{
}
$ b $ / **
*用代码覆盖反序列化}，然后将其解析为
*应用程序类型。
* /
protected void afterRead（JsonElement反序列化）{
}
}
然后为所有需要考虑的类创建一个子类。您必须为每个包含long（在本例中）的类创建一个。但是你不需要序列化任何东西，除了长整型值（以及任何其他特定于bson的值）以外。
public class MyTestObjectTypeAdapterFactory extends CustomizedTypeAdapterFactory< ; MyTestObject>
{
public MyTestObjectTypeAdapterFactory（）
{
super（MyTestObject.class）;
}
@Override
protected void beforeWrite（MyTestObject source，JsonElement toSerialize）
{
//你可以在这里转换其他方式，I让mongo的文档解析器来处理这个问题。
}
@Override
protected void afterRead（JsonElement反序列化）
{
JsonObject timestamp = deserialized.getAsJsonObject（）。get（timestamp） .getAsJsonObject（）;
deserialized.getAsJsonObject（）。remove（timestamp）;
deserialized.getAsJsonObject（）。add（timestamp，timestamp.get（$ numberLong））;
然后生成Gson：
Gson gson = new GsonBuilder（）。registerTypeAdapterFactory（new MyTestObjectTypeAdapterFactory（））。create（）;
I'm looking for a way to read a MongoDB document into a POJO using GSON. It works just fine until you run into stuff like date's and longs.
I would like to write a custom adapter for Gson which will convert any BSON encoded long. Reading this post I have created my own adapter:
public class BsonLongTypeAdapter extends TypeAdapter<Long>
{
@Override
public void write(JsonWriter out, Long value) throws IOException
{
out.beginObject()
.name("$numberLong")
.value(value.toString())
.endObject();
}
@Override
public Long read(JsonReader in) throws IOException
{
in.beginObject();
assert "$numberLong".equals(in.nextName());
Long value = in.nextLong();
in.endObject();
return value;
}
}
I have defined the following tests to check if this works:
@Test
public void canWriteCorrectJSON() {
Gson gson = new GsonBuilder().registerTypeAdapter(Long.class, new BsonLongTypeAdapter()).create();
MyTestObject obj = new MyTestObject(1458569479431L);
String gsonString = gson.toJson(obj);
assertEquals("{\"timestamp\":{\"$numberLong\":\"1458569479431\"}}",gsonString);
}
@Test
public void canReadFromJSON() {
Gson gson = new GsonBuilder().registerTypeAdapter(Long.class, new BsonLongTypeAdapter()).create();
MyTestObject actualTaskObject = gson.fromJson("{\"timestamp\":{\"$numberLong\":\"1458569479431\"}}", MyTestObject.class);
MyTestObject taskObject = new MyTestObject(1458569479431L);
assertEquals(taskObject.getTimestamp(),actualTaskObject.getTimestamp());
}
private static class MyTestObject
{
long timestamp;
public MyTestObject(long ts)
{
timestamp = ts;
}
public long getTimestamp()
{
return timestamp;
}
public void setTimestamp(long timestamp)
{
this.timestamp = timestamp;
}
}
The first (write) test works just fine, but the read test fails on:
com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a long but was BEGIN_OBJECT at line 1 column 15 path $.timestamp
Because the read function from my adapter is never called. I presume this might be because I want to map to MyTestObject and not to Long, but I don't want to have to write adapters for all classes that contain longs.
Is it possible to write an adapter for GSON that converts all BSON longs I send into it?
解决方案
I solved it using a CustomizedTypeAdapterFactory. See this question
Basically first write a customized adapter:
public abstract class CustomizedTypeAdapterFactory<C>
implements TypeAdapterFactory
{
private final Class<C> customizedClass;
public CustomizedTypeAdapterFactory(Class<C> customizedClass) {
this.customizedClass = customizedClass;
}
@SuppressWarnings("unchecked") // we use a runtime check to guarantee that 'C' and 'T' are equal
public final <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
return type.getRawType() == customizedClass
? (TypeAdapter<T>) customizeMyClassAdapter(gson, (TypeToken<C>) type)
: null;
}
private TypeAdapter<C> customizeMyClassAdapter(Gson gson, TypeToken<C> type) {
final TypeAdapter<C> delegate = gson.getDelegateAdapter(this, type);
final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
return new TypeAdapter<C>() {
@Override public void write(JsonWriter out, C value) throws IOException
{
JsonElement tree = delegate.toJsonTree(value);
beforeWrite(value, tree);
elementAdapter.write(out, tree);
}
@Override public C read(JsonReader in) throws IOException {
JsonElement tree = elementAdapter.read(in);
afterRead(tree);
return delegate.fromJsonTree(tree);
}
};
}
/**
* Override this to muck with {@code toSerialize} before it is written to
* the outgoing JSON stream.
*/
protected void beforeWrite(C source, JsonElement toSerialize) {
}
/**
* Override this to muck with {@code deserialized} before it parsed into
* the application type.
*/
protected void afterRead(JsonElement deserialized) {
}
}
And then create a subclass for all classes that need to be taken into account. You do have to create one for every class containing a long (in this case). But you don't have to serialize anything but the long value (and any other bson specific values)
public class MyTestObjectTypeAdapterFactory extends CustomizedTypeAdapterFactory<MyTestObject>
{
public MyTestObjectTypeAdapterFactory()
{
super(MyTestObject.class);
}
@Override
protected void beforeWrite(MyTestObject source, JsonElement toSerialize)
{
//you could convert back the other way here, I let mongo's document parser take care of that.
}
@Override
protected void afterRead(JsonElement deserialized)
{
JsonObject timestamp = deserialized.getAsJsonObject().get("timestamp").getAsJsonObject();
deserialized.getAsJsonObject().remove("timestamp");
deserialized.getAsJsonObject().add("timestamp",timestamp.get("$numberLong"));
}
}
and then generate Gson with:
Gson gson = new GsonBuilder().registerTypeAdapterFactory(new MyTestObjectTypeAdapterFactory()).create();
这篇关于使用GSON和TypeAdapter将BSON（mongoDB）读入POJO的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
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