java - 如何反序列化大 JSON 文件 (~300Mb)

Question

我想解析一个 JSON 文件(大小 ~300Mb)。我使用 Jackson 库和 ObjectMapper。如果我出现内存问题，这正常吗？

第一次，我使用BufferedReader，它使应用程序崩溃。接下来，我使用这个库。解析并保存到SQLite数据库需要多少时间，很长吗？

Answer 1

jackson

您可以将Streaming API与常规ObjectMapper混合使用。使用这些我们可以实现很好的Iterator类(class)。使用 URL 我们可以构建流并传递给我们的实现。示例代码如下所示:

import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;
import java.math.BigDecimal;
import java.net.URL;
import java.util.Iterator;

public class JsonPathApp {

    public static void main(String[] args) throws Exception {
        //Just to make it work. Probably you should not do that!
        SSLUtilities.trustAllHostnames();
        SSLUtilities.trustAllHttpsCertificates();

        URL url = new URL("https://data.opendatasoft.com/explore/dataset/vehicules-commercialises@public/download/?format=json&timezone=Europe/Berlin");
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(url.openConnection().getInputStream()))) {
            FieldsJsonIterator fieldsJsonIterator = new FieldsJsonIterator(reader);
            while (fieldsJsonIterator.hasNext()) {
                Fields fields = fieldsJsonIterator.next();
                System.out.println(fields);
                // Save object to DB
            }
        }
    }
}

class FieldsJsonIterator implements Iterator<Fields> {

    private final ObjectMapper mapper;
    private final JsonParser parser;

    public FieldsJsonIterator(Reader reader) throws IOException {
        mapper = new ObjectMapper();
        mapper.disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

        parser = mapper.getFactory().createParser(reader);
        skipStart();
    }

    private void skipStart() throws IOException {
        while (parser.currentToken() != JsonToken.START_OBJECT) {
            parser.nextToken();
        }
    }

    @Override
    public boolean hasNext() {
        try {
            while (parser.currentToken() == null) {
                parser.nextToken();
            }
        } catch (IOException e) {
            throw new IllegalStateException(e);
        }

        return parser.currentToken() == JsonToken.START_OBJECT;
    }

    @Override
    public Fields next() {
        try {
            return mapper.readValue(parser, FieldsWrapper.class).fields;
        } catch (IOException e) {
            throw new IllegalStateException(e);
        }
    }

    private static final class FieldsWrapper {
        public Fields fields;
    }
}

class Fields {

    private String cnit;

    @JsonProperty("puissance_maximale")
    private BigDecimal maximumPower;

    @JsonProperty("champ_v9")
    private String fieldV9;

    @JsonProperty("boite_de_vitesse")
    private String gearbox;

    // add other required properties

    // getters, setters, toString
}

上面的代码打印:

Fields{cnit='MMB76K3BQJ41', maximumPower=110.0, fieldV9='70/220*2006/96EURO4', gearbox='A 5'}
Fields{cnit='M10MCDVPF15Z219', maximumPower=95.0, fieldV9='"715/2007*566/2011EURO5', gearbox='A 7'}
Fields{cnit='M10MCDVP027V654', maximumPower=150.0, fieldV9='715/2007*692/2008EURO5', gearbox='A 7'}
Fields{cnit='M10MCDVPG137264', maximumPower=120.0, fieldV9='715/2007*692/2008EURO5', gearbox='M 6'}
Fields{cnit='MVV4912QN718', maximumPower=210.0, fieldV9='null', gearbox='A 6'}
Fields{cnit='MMB76K3B2K88', maximumPower=110.0, fieldV9='null', gearbox='A 5'}
Fields{cnit='M10MCDVP012N140', maximumPower=80.0, fieldV9='70/220*2006/96EURO4', gearbox='M 6'}
Fields{cnit='MJN5423PU123', maximumPower=88.0, fieldV9='null', gearbox='M 6'}
Fields{cnit='M10MCDVP376T303', maximumPower=120.0, fieldV9='"715/2007*692/2008EURO5', gearbox='M 6'}
Fields{cnit='MMB53H3B5Z93', maximumPower=80.0, fieldV9='70/220*2006/96EURO4', gearbox='M 6'}
Fields{cnit='MPE1403E4834', maximumPower=81.0, fieldV9='null', gearbox='M 5'}
Fields{cnit='M10MCDVP018J905', maximumPower=110.0, fieldV9='70/220*2006/96EURO4', gearbox='M 6'}
Fields{cnit='M10MCDVPG112904', maximumPower=100.0, fieldV9='"715/2007*692/2008EURO5', gearbox='M 6'}
Fields{cnit='M10MCDVP015R723', maximumPower=110.0, fieldV9='70/220*2006/96EURO4', gearbox='A 5'}
...

Gson

我们可以使用Gson做同样的事情。示例实现如下所示:

class FieldsJsonIterator implements Iterator<Fields> {

    private final Gson mapper;
    private final JsonReader parser;

    public FieldsJsonIterator(Reader reader) throws IOException {
        mapper = new GsonBuilder().create();

        parser = mapper.newJsonReader(reader);
        skipStart();
    }

    private void skipStart() throws IOException {
        parser.beginArray();
    }

    @Override
    public boolean hasNext() {
        try {
            return parser.hasNext();
        } catch (IOException e) {
            throw new IllegalStateException(e);
        }
    }

    @Override
    public Fields next() {
        return ((FieldsWrapper) mapper.fromJson(parser, FieldsWrapper.class)).fields;
    }

    private static final class FieldsWrapper {
        public Fields fields;
    }
}

class Fields {

    private String cnit;

    @SerializedName("puissance_maximale")
    private BigDecimal maximumPower;

    @SerializedName("champ_v9")
    private String fieldV9;

    @SerializedName("boite_de_vitesse")
    private String gearbox;

    // getters, setters, toString
}

用法和输出应该与 Jackson 相同。

另请参阅:

• Best way to access nested JSON objects with Java
• Whats an easy way to totally ignore ssl with java url connections?