写一个大数据hdfs小文件合并的java代码，包括临时文件处理，完成hdfs的小文件治理

以下是一个使用Hadoop MapReduce框架实现HDFS小文件合并的Java代码：

package com.example.hadoop;

import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.Reducer;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;

import java.io.IOException;

public class HDFSFileMerging {

public static class FileMapper extends Mapper<Object, Text, Text, Text> {
private Text filenameKey = new Text();

@Override
protected void map(Object key, Text value, Context context) throws IOException, InterruptedException {
String filename = value.toString();
filenameKey.set(filename);
context.write(filenameKey, new Text(""));
}
}

public static class FileReducer extends Reducer<Text, Text, Text, Text> {
private FileSystem fs;
private Path tempDir;
private Path outputDir;

@Override
protected void setup(Context context) throws IOException, InterruptedException {
Configuration conf = context.getConfiguration();
fs = FileSystem.get(conf);
tempDir = new Path(conf.get("tempDir"));
outputDir = new Path(conf.get("outputDir"));
if (fs.exists(outputDir)) {
fs.delete(outputDir, true);
}
fs.mkdirs(tempDir);
}

@Override
protected void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
String filename = key.toString();
Path inputFile = new Path(filename);
Path tempFile = new Path(tempDir, filename);
fs.copyFromLocalFile(inputFile, tempFile);
}

@Override
protected void cleanup(Context context) throws IOException, InterruptedException {
Path[] tempFiles = fs.listStatus(tempDir);
for (Path tempFile : tempFiles) {
Path outputFile = new Path(outputDir, tempFile.getName());
fs.rename(tempFile, outputFile);
}
fs.delete(tempDir, true);
}
}

public static void main(String[] args) throws Exception {
Configuration conf = new Configuration();
Job job = Job.getInstance(conf, "HDFS File Merging");
job.setJarByClass(HDFSFileMerging.class);
job.setMapperClass(FileMapper.class);
job.setReducerClass(FileReducer.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(Text.class);
FileInputFormat.addInputPath(job, new Path(args[0]));
Path outputDir = new Path(args[1]);
job.getConfiguration().set("tempDir", outputDir + "/temp");
job.getConfiguration().set("outputDir", outputDir.toString());
FileOutputFormat.setOutputPath(job, outputDir);
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}

在这个例子中，我们定义了一个FileMapper类来读取HDFS上的小文件名并将其作为键传递给FileReducer。在FileReducer中，我们创建一个临时目录来存储所有小文件，并将它们从HDFS上的输入路径中复制到临时目录中。最后，我们将所有临时文件移动到输出目录中，并删除临时目录。请注意，这个例子中的输出目录必须是空的。