Hadoop的序列化与反序列化实操

0x00 文章内容

1. 编写代码
2. 测试结果

0x01 编写代码

前提：因为需要用到Hadoop，所以需要先引入Hadoop相关的jar包

<dependency>
    <groupId>org.apache.hadoop</groupId>
    <artifactId>hadoop-client</artifactId>
    <version>2.7.5</version>
</dependency>

1. 编写对象类

a. 编写Block类

package com.shaonaiyi.hadoop.serialize;

import org.apache.hadoop.io.Writable;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

/**
 * @Author [e-mail]
 * @Date 2019/12/13 16:51
 * @Description 定义引入Hadoop序列化机制的BlockWritable类
 */
public class BlockWritable implements Writable {

    private long blockId;
    private long numBytes;
    private long generationStamp;

    public BlockWritable(long blockId) {
        this.blockId = blockId;
    }

    public BlockWritable(long blockId, long numBytes, long generationStamp) {
        this.blockId = blockId;
        this.numBytes = numBytes;
        this.generationStamp = generationStamp;
    }

    public long getBlockId() {
        return blockId;
    }

    public long getNumBytes() {
        return numBytes;
    }

    public long getGenerationStamp() {
        return generationStamp;
    }

    @Override
    public String toString() {
        return "Block{" +
                "blockId=" + blockId +
                ", numBytes=" + numBytes +
                ", generationStamp=" + generationStamp +
                '}';
    }

    public void write(DataOutput dataOutput) throws IOException {
        dataOutput.writeLong(blockId);
        dataOutput.writeLong(numBytes);
        dataOutput.writeLong(generationStamp);
    }

    public void readFields(DataInput dataInput) throws IOException {
        this.blockId = dataInput.readLong();
        this.numBytes = dataInput.readLong();
        this.generationStamp = dataInput.readLong();
    }

}

2. 编写调用测试代码

a. 编写序列化与反序列化代码

package com.shaonaiyi.hadoop.serialize;

import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableFactories;
import java.io.*;

/**
 * @Author [e-mail]
 * @Date 2019/12/13 16:53
 * @Description 编写Hadoop序列化机制测试类代码
 */
public class HadoopSerializableTest {

    public static void main(String[] args) throws IOException {
        String fileName = "blockWritable.txt";

        serialize(fileName);
//        deSerialize(fileName);

    }

    private static void serialize(String fileName) throws IOException {
        BlockWritable block = new BlockWritable(78062621L, 39447651L, 56737546L);

        File file = new File(fileName);
        if (file.exists()) {
            file.delete();
        }
        file.createNewFile();

        FileOutputStream fileOutputStream = new FileOutputStream(file);
        DataOutputStream dataOutputStream = new DataOutputStream(fileOutputStream);
        block.write(dataOutputStream);
        dataOutputStream.close();
    }

    private static void deSerialize(String fileName) throws IOException {

        FileInputStream fileInputStream = new FileInputStream(fileName);
        DataInputStream dataInputStream = new DataInputStream(fileInputStream);
        Writable writable = WritableFactories.newInstance(BlockWritable.class);
        writable.readFields(dataInputStream);
        System.out.println((BlockWritable)writable);

    }

}

0x02 测试结果

1. 测试序列化

a. 执行序列化方法，发现项目与main文件夹同级目录下多了一个blockWritable.txt文件，双击打开：

查看大小，发现是24字节：
我们定义的BlockWritable类，Long类是8个字节，三个Long类型的属性是24个字节，没有引入额外的信息，是我们想要的结果，请与上一篇教程做比较：Java的序列化与反序列化实操。

2. 测试反序列化

a. 打开反序列化方法，执行：

//        serialize(fileName);
        deSerialize(fileName);

发现可以将BlockWritable对象反序列化出来了。

3. 测试修改对象代码

a. 尝试修改BlockWritable类，如在里面添加一个无参数的构造方法

    public BlockWritable() {
    
    }

b. 然后再反序列化，发现执行的结果并没有报错，与前面一样。

0xFF 总结

1. Hadoop的序列化机制解决了内置的Java序列化接口的缺陷。
2. 本教程拥有前置教程：Java的序列化与反序列化实操！
3. Hadoop的序列化机制接口如下：

Java类型	Writable	序列化后字节大小
boolean	BooleanWritable	1
byte	ByteWritable	1
int	IntWritable	4
float	FloatWriable	4
long	LongWritable	8
double	DoubleWritable	8
String	Text	据具体情况而定

4. 在平时代码中，可以参考如下：

 Text text = new Text();
 String word = "hello";
 text.set(word);

 IntWritable intWritable = new IntWritable(3);

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作者简介：邵奈一
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