Gson Streaming APIs to read and write JSON
Streaming APIs are efficient ways of processing large JSON files or data without loading the entire document into memory. Gson provides JsonReader and JsonWriter classes for JSON streaming.
Gson’s Streaming API
JsonWriter– Write JSON as a stream.JsonReader– Read JSON as a stream.
Table of contents:
- 1. Setup Google Gson
- 1. Write JSON String using JsonWriter
- 2. Write JSON Array using JsonWriter
- 3. Read JSON String using JsonReader
- 4. Read JSON Array using JsonReader
- 5. Download Source Code
- 6. References
P.S Tested with Gson 2.10.1
1. Setup Google Gson
Declares gson in the pom.xml.
pom.xml
<dependency>
<groupId>com.google.code.gson</groupId>
<artifactId>gson</artifactId>
<version>2.10.1</version>
</dependency>
1. Write JSON String using JsonWriter
The following example uses Gson’s JsonWriter to write JSON to a file named user.json.
JsonWriterExample1.java
package com.mkyong.json.gson.streaming;
import com.google.gson.stream.JsonWriter;
import java.io.FileWriter;
import java.io.IOException;
public class JsonWriterExample1 {
public static void main(String[] args) {
try (JsonWriter writer = new JsonWriter(new FileWriter("user.json"))) {
// pretty print
writer.setIndent(" ");
writer.beginObject(); // {
writer.name("name").value("mkyong"); // "name" : "mkyong"
writer.name("age").value(42); // "age" : 42
writer.name("messages"); // "messages" :
writer.beginArray(); // [
writer.value(1); // 1,
writer.value(2); // 2,
writer.value(3); // 3
writer.endArray(); // ]
writer.endObject(); // }
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
Output
user.json
{
"name": "mkyong",
"age": 42,
"messages": [
1,
2,
3
]
}
2. Write JSON Array using JsonWriter
The following example uses Gson’s JsonWriter to write a JSON Array to a file named user.json.
JsonWriterExample2.java
package com.mkyong.json.gson.streaming;
import com.google.gson.stream.JsonWriter;
import java.io.FileWriter;
import java.io.IOException;
public class JsonWriterExample2 {
public static void main(String[] args) {
try (JsonWriter writer = new JsonWriter(new FileWriter("user.json"))) {
// pretty print
writer.setIndent(" ");
writer.beginArray(); // [
writer.beginObject(); // {
writer.name("name").value("mkyong"); // "name" : "mkyong"
writer.name("age").value(42); // "age" : 42
writer.name("messages"); // "messages" :
writer.beginArray(); // [
writer.value(1); // 1,
writer.value(2); // 2,
writer.value(3); // 3
writer.endArray(); // ]
writer.endObject(); // }
writer.beginObject(); // {
writer.name("name").value("ah pig"); // "name" : "ah pig"
writer.name("age").value(20); // "age" : 20
writer.name("messages"); // "messages" :
writer.beginArray(); // [
writer.value("a"); // a,
writer.value("b"); // b,
writer.value("c"); // c
writer.endArray(); // ]
writer.endObject(); // }
writer.endArray(); // [
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
Output
user.json
[
{
"name": "mkyong",
"age": 42,
"messages": [
1,
2,
3
]
},
{
"name": "ah pig",
"age": 20,
"messages": [
"a",
"b",
"c"
]
}
]
3. Read JSON String using JsonReader
The following example uses Gson’s JsonReader to read JSON from a file named user.json and print out the JSON values.
A JSON object.
user.json
{
"name": "mkyong",
"age": 42,
"messages": [
1,
2,
3
]
}
JsonReaderExample1.java
package com.mkyong.json.gson.streaming;
import com.google.gson.stream.JsonReader;
import java.io.FileReader;
import java.io.IOException;
public class JsonReaderExample1 {
public static void main(String[] args) {
try (JsonReader reader = new JsonReader(new FileReader("user.json"))) {
reader.beginObject();
while (reader.hasNext()) {
String name = reader.nextName();
switch (name) {
case "name" -> System.out.println(reader.nextString());
case "age" -> System.out.println(reader.nextInt());
case "messages" -> {
// read array
reader.beginArray();
while (reader.hasNext()) {
System.out.println(reader.nextString());
}
reader.endArray();
}
default -> reader.skipValue(); // skip others
}
}
reader.endObject();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
Output
mkyong
42
1
2
3
4. Read JSON Array using JsonReader
The following example uses Gson’s JsonReader to read JSON Array from a file named user.json and print out the JSON values.
A JSON Array.
user.json
[
{
"name": "mkyong",
"age": 42,
"messages": [
1,
2,
3
]
},
{
"name": "ah pig",
"age": 20,
"messages": [
"a",
"b",
"c"
]
}
]
JsonReaderExample2.java
package com.mkyong.json.gson.streaming;
import com.google.gson.stream.JsonReader;
import java.io.FileReader;
import java.io.IOException;
public class JsonReaderExample2 {
public static void main(String[] args) {
try (JsonReader reader = new JsonReader(new FileReader("user.json"))) {
reader.beginArray(); // this is an json array
while (reader.hasNext()) {
reader.beginObject();
while (reader.hasNext()) {
String name = reader.nextName();
switch (name) {
case "name" -> System.out.println(reader.nextString());
case "age" -> System.out.println(reader.nextInt());
case "messages" -> {
// read array
reader.beginArray();
while (reader.hasNext()) {
System.out.println(reader.nextString());
}
reader.endArray();
}
default -> reader.skipValue(); // skip others
}
}
reader.endObject();
}
reader.endArray();
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
Output
mkyong
42
1
2
3
ah pig
20
a
b
c
5. Download Source Code
$ git clone https://github.com/mkyong/java-json
$ cd gson
6. References
About Author
Founder of Mkyong.com, love Java and open source stuff. Follow him on Twitter. If you like my tutorials, consider make a donation to these charities.
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