我有一个非常大的 > 1GB 的 JSON 文件,其中包含一个数组(它是 secret 的,但这个 sleep 持续时间数据文件是一个代理:)
[
{
"date": "August 17, 2015",
"hours": 7,
"minutes": 10
},
{
"date": "August 19, 2015",
"hours": 4,
"minutes": 46
},
{
"date": "August 19, 2015",
"hours": 7,
"minutes": 22
},
{
"date": "August 21, 2015",
"hours": 4,
"minutes": 48
},
{
"date": "August 21, 2015",
"hours": 6,
"minutes": 1
}
]
我使用 JSON2POJO 生成了一个“ sleep ”对象定义。
现在,可以使用 Jackson 的 Mapper 转换为数组,然后使用 Arrays.stream(ARRAY)。除了这会崩溃(是的,这是一个大文件)。
显而易见的是使用 Jackson 的 Streaming API。但那是超低水平。特别是,我仍然想要 sleep 对象。
如何使用 Jackson Streaming JSON 阅读器和我的 Sleep.java 类生成 Java 8 sleep 对象流?
最佳答案
我找不到很好的解决方案,我需要一个针对特定情况的解决方案: 我有一个 >1GB 的 JSON 文件(一个顶级 JSON 数组,包含数万个大对象),并且在访问生成的 Java 对象数组时使用普通的 Jackson 映射器导致崩溃。
我找到的使用 Jackson Streaming API 的示例丢失了如此吸引人的对象映射,当然也不允许通过(显然合适的)Java 8 Streaming API 访问对象。
这是一个简单的使用示例:
//Use the JSON File included as a resource
ClassLoader classLoader = SleepReader.class.getClassLoader();
File dataFile = new File(classLoader.getResource("example.json").getFile());
//Simple example of getting the Sleep Objects from that JSON
new JsonArrayStreamDataSupplier<>(dataFile, Sleep.class) //Got the Stream
.forEachRemaining(nightsRest -> {
System.out.println(nightsRest.toString());
});
这是来自 example.json 的一些 JSON
[
{
"date": "August 17, 2015",
"hours": 7,
"minutes": 10
},
{
"date": "August 19, 2015",
"hours": 4,
"minutes": 46
},
{
"date": "August 19, 2015",
"hours": 7,
"minutes": 22
},
{
"date": "August 21, 2015",
"hours": 4,
"minutes": 48
},
{
"date": "August 21, 2015",
"hours": 6,
"minutes": 1
}
]
如果你不想去 GitHub(你应该去),这里是包装类本身:
/**
* @license APACHE LICENSE, VERSION 2.0 http://www.apache.org/licenses/LICENSE-2.0
* @author Michael Witbrock
*/
package com.michaelwitbrock.jacksonstream;
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.File;
import java.io.IOException;
import java.util.Iterator;
import java.util.Spliterators;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;
public class JsonArrayStreamDataSupplier<T> implements Iterator<T> {
/*
* This class wraps the Jackson streaming API for arrays (a common kind of
* large JSON file) in a Java 8 Stream. The initial motivation was that
* use of a default objectmapper to a Java array was crashing for me on
* a very large JSON file (> 1GB). And there didn't seem to be good example
* code for handling Jackson streams as Java 8 streams, which seems natural.
*/
static ObjectMapper mapper = new ObjectMapper();
JsonParser parser;
boolean maybeHasNext = false;
int count = 0;
JsonFactory factory = new JsonFactory();
private Class<T> type;
public JsonArrayStreamDataSupplier(File dataFile, Class<T> type) {
this.type = type;
try {
// Setup and get into a state to start iterating
parser = factory.createParser(dataFile);
parser.setCodec(mapper);
JsonToken token = parser.nextToken();
if (token == null) {
throw new RuntimeException("Can't get any JSON Token from "
+ dataFile.getAbsolutePath());
}
// the first token is supposed to be the start of array '['
if (!JsonToken.START_ARRAY.equals(token)) {
// return or throw exception
maybeHasNext = false;
throw new RuntimeException("Can't get any JSON Token fro array start from "
+ dataFile.getAbsolutePath());
}
} catch (Exception e) {
maybeHasNext = false;
}
maybeHasNext = true;
}
/*
This method returns the stream, and is the only method other
than the constructor that should be used.
*/
public Stream<T> getStream() {
return StreamSupport.stream(Spliterators.spliteratorUnknownSize(this, 0), false);
}
/* The remaining methods are what enables this to be passed to the spliterator generator,
since they make it Iterable.
*/
@Override
public boolean hasNext() {
if (!maybeHasNext) {
return false; // didn't get started
}
try {
return (parser.nextToken() == JsonToken.START_OBJECT);
} catch (Exception e) {
System.out.println("Ex" + e);
return false;
}
}
@Override
public T next() {
try {
JsonNode n = parser.readValueAsTree();
//Because we can't send T as a parameter to the mapper
T node = mapper.convertValue(n, type);
return node;
} catch (IOException | IllegalArgumentException e) {
System.out.println("Ex" + e);
return null;
}
}
}
关于java - 如何将 JSON 文件转换为 Java 8 对象流?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/35000998/