java 读取一个大的json文件，但是懒得这么做，以免使用太多内存。感谢Java 8流，这很有可能

Posted 2021-05-08

Located here: https://gist.github.com/smac89/15fc3bffa4f965d18587eec2db4972dd

public class Student {
    public final String studentId;
    public final String name;
    public final Gender gender;
    public final String email;
    public final Integer numberOfClasses;

    public Student(String studentId, String name, Gender gender, String email, Integer numberOfClasses) {
        this.studentId = studentId;
        this.name = name;
        this.gender = gender;
        this.email = email;
        this.numberOfClasses = numberOfClasses;
    }

    @Override
    public String toString() {
        return "Name: " + name + ", ID: " + studentId + ", Gender: " + gender.name() +
                ", email: " + email + ", Class count: " + String.valueOf(numberOfClasses);
    }

    public enum Gender {
        Male, Female
    }
}

import com.google.gson.Gson;
import com.google.gson.stream.JsonReader;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.reflect.Type;
import java.net.URL;
import java.util.Iterator;
import java.util.Spliterators;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public class ReadJsonFile {
    private static final class JsonIterator<T> implements Iterator<T> {
        private final Gson gson;
        private final Type objectType;
        private final JsonReader reader;

        private JsonIterator(JsonReader reader, Gson gson, Type objectType) {
            this.gson = gson;
            this.objectType = objectType;
            this.reader = reader;
        }

        @Override
        public boolean hasNext() {
            try {
                return reader.hasNext();
            } catch (IOException ioe) {
                return false;
            }
        }

        @Override
        public T next() {
            return gson.fromJson(reader, objectType);
        }
    }

    public static <J> Stream<J> readJsonFromFile(Gson gson, URL jsonFile, Type type) throws IOException {
        JsonReader reader = new JsonReader(
                new BufferedReader(new InputStreamReader(jsonFile.openStream())));
        reader.beginArray();
        if (!reader.hasNext()) {
            return Stream.empty();
        }
        return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
                new JsonIterator<J>(reader, gson, type), 0), false).onClose(() -> {
            try {
                reader.close();
            } catch (IOException ioe) {
            }
        });
    }
}

import com.google.common.io.Resources;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonObject;

import java.io.IOException;
import java.net.URL;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.stream.Stream;

public class Main {

    public static void main(String args[]) {
        URL data = Resources.getResource("data.json");

        GsonBuilder builder = new GsonBuilder();
        builder.registerTypeAdapter(Student.class, (JsonDeserializer<Student>) (json, typeOfT, context) -> {
            JsonObject object = json.getAsJsonObject();
            Student.Gender gender = Student.Gender.valueOf(object.get("gender").getAsString());
            String firstName = object.get("first_name").getAsString();
            String lastName = object.get("last_name").getAsString();
            String email = object.get("email").getAsString();
            String Id = object.get("id").getAsString();
            int numberOfClasses = object.get("class_count").getAsInt();

            return new Student(Id, firstName + " " + lastName, gender, email, numberOfClasses);
        });

        try (Stream<Student> studentStream = ReadJsonFile.readJsonFromFile(builder.create(), data,
                                                                           Student.class)) {
            studentStream.forEach(System.out::println);
        } catch (IOException ioe) {
            Logger.getAnonymousLogger().log(Level.WARNING, ioe,
                                            () -> "An exception occurred when reading the json");
        }
    }
}