如何将 JSON 文件转换为 Java 8 对象流？

Question

我有一个非常大的 > 1GB JSON 文件包含一个数组（它是机密的，但这个睡眠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
        }
    ]

我已经使用 JSON2POJO 生成了“睡眠”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's Streaming API。但那是超低水平。特别是，我仍然想要睡眠对象。

如何使用 Jackson Streaming JSON reader 和我的 Sleep.java class 生成 Java 8 睡眠对象流？

Answer 1

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 JSON 文件（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 访问对象流式传输 API。

The code is now on GitHub

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
    }
]

而且，如果你不想去 GitHub（你应该），这里是包装器 class 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;
        }

    }


}

Answer 2

删除迭代器的实现

我认为您可以使用 Jackson 的 API 摆脱整个 Iterator 实现。

这里的问题 22 是 readValueAs 可以 return 一个迭代器，我唯一没有完全弄清楚的是为什么我必须先消耗 JSON 数组开始才能让 Jackson 做工作正常

public class InputStreamJsonArrayStreamDataSupplier<T> implements Supplier<Stream<T>> {


private ObjectMapper mapper = new ObjectMapper();
private JsonParser jsonParser;
private Class<T> type;



public InputStreamJsonArrayStreamDataSupplier(Class<T> type) throws IOException {
    this.type = type;

    // Setup and get into a state to start iterating
    jsonParser = mapper.getFactory().createParser(data);
    jsonParser.setCodec(mapper);
    JsonToken token = jsonParser.nextToken();
    if (JsonToken.START_ARRAY.equals(token)) {
        // if it is started with START_ARRAY it's ok
        token = jsonParser.nextToken();
    }
    if (!JsonToken.START_OBJECT.equals(token)) {
        throw new RuntimeException("Can't get any JSON object from input " + data);
    }
}


public Stream<T> get() {
    try {
        return StreamSupport.stream(Spliterators.spliteratorUnknownSize((Iterator<T>) jsonParser.readValuesAs(type), 0), false);
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
}

如何将 JSON 文件转换为 Java 8 对象流？

How do I turn a JSON file into a Java 8 Object Stream?

java

arrays

json

bigdata

java-8

删除迭代器的实现