用socket写了一个用传输文件的工具，可以正常传输文件，可以按文件大小写入了，第一张图片可以看，第二张开始看不到了。

接收端

package com.example.socketserivce.activity;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.telephony.mbms.FileInfo;
import android.util.Log;

import com.example.socketserivce.R;

import java.io.BufferedInputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;

public class ReceiveActivity extends AppCompatActivity {
    public static final String TAG="ReceiveActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_receive);


        new Thread (){
            @Override
            public void run() {
                startSocket();
            }
        }.start ();

    }

    private void startSocket() {
        String path="/data/data/com.example.socketserivce/cache/Success-Import";
        int  fileNum=0;
        long totalSize=0;

        //接收信息
        try {
            File dirs1=new File (path);
            Log.d (TAG, "本机地址: "+InetAddress.getLocalHost ());
            if (dirs1.exists ()){
            ServerSocket serverSocket=new ServerSocket (8888);
            Socket socket=serverSocket.accept ();
            Log.d (TAG, "成功监听到端口: ");
            DataInputStream din=new DataInputStream (new BufferedInputStream (socket.getInputStream ()));
            //获取文件的个数
            fileNum=din.readInt ();
            Log.d (TAG, "startSocket: "+fileNum);

            //创建数组将文件名和文件大小保存正在其中
            String[] mFileName=new String[fileNum];
            long mFileSize[]=new long[fileNum];
            //读取对应的文件名和文件大小
            for (int i=0;i<fileNum;i++){
                mFileName[i]=din.readUTF ();
                mFileSize[i]=din.readLong ();
                Log.d (TAG, "文件名和文件大小: "+mFileName[i]+mFileSize[i]+"个数："+i);
            }
            totalSize=din.readLong ();
                Log.d (TAG, "totalSize: "+totalSize);

            int leftLen = 0; // 写满文件后缓存区中剩余的字节长度。
            int bufferedLen = 0; // 当前缓冲区中的字节数
            int writeLen = 0; // 每次向文件中写入的字节数
            long writeLens = 0; // 当前已经向单个文件中写入的字节总数
            long totalWriteLens=0;//写入的所有字节数
            byte buf[]=new byte[8192];

            for (int i=0;i<fileNum;i++){
                writeLens=0;//每次循环将 当前已经写入字节数字清零
                //文件写出，文件写出位置
                FileOutputStream fout=new FileOutputStream ("/data/data/com.example.socketserivce/cache/Save/"+mFileName[i]);
                while (true){
                    if (leftLen>0){//从第二个开始,当前缓冲区的字节数为leftLen
                        bufferedLen=leftLen;
                    }else{
                        bufferedLen=din.read (buf);
                        Log.d (TAG, "bufferedLen: "+bufferedLen);
                    }
                    if (bufferedLen==-1) return;
                    Log.d (TAG, "startSocket: "+writeLens+bufferedLen+mFileSize[i]);
                    if (writeLens+bufferedLen>=mFileSize[i]){
                        Log.d (TAG, "开始传输: "+"writeLens+bufferedLen"+(writeLens+bufferedLen)+"文件大小："+mFileSize[i]+"buf:"+buf+"writeLen:"+writeLen);
                        leftLen= (int) (writeLens+bufferedLen-mFileSize[i]);
                        writeLen=bufferedLen-leftLen;
                        fout.write (buf,0,writeLen);
                        totalWriteLens+=writeLen;
                        move (buf,writeLen,leftLen);
                        break;
                    }else {
                        Log.d (TAG, "全部传输: ");
                        fout.write (buf,0   ,bufferedLen);
                        fout.flush ();
                        writeLens+=bufferedLen;
                        totalWriteLens+=bufferedLen;
                        if (totalWriteLens>=totalSize){
                            Log.d (TAG, "totalWriteLens>=totalSize: ");
                            return;
                        }
                        leftLen=0;
                    }

                }
                Log.d (TAG, "传输已完成: ");
                fout.close ();

                Log.d (TAG, "totalWriteLens: "+totalWriteLens);
            }
                socket.close ();
            }


        } catch (IOException e) {
            e.printStackTrace ();
        }
    }

    /**
     *
     * @param buf 读取大小
     * @param writeLen   已经写入到文件中的
     * @param leftLen    内存中剩余的字节长度
     */
    public static void move( byte buf[] ,int writeLen, int leftLen){
        for (int i = 0; i < leftLen; i++) {
            if(buf[writeLen+i] != 0){
                buf[i] = buf[writeLen+i];
            }
        }
    }
}

传输端

package com.example.socketserivce.activity;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;

import com.example.socketserivce.R;

import java.io.BufferedInputStream;
import java.io.DataOutput;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;

public class SendActivity extends AppCompatActivity {
    public static final String TAG="SendActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate (savedInstanceState);
        setContentView (R.layout.activity_send);


        new Thread (){
            @Override
            public void run() {
                startSocket();
                try {
                    Log.d (TAG, "本机地址: "+InetAddress.getLocalHost ());
                } catch (UnknownHostException e) {
                    e.printStackTrace ();
                }
            }
        }.start ();


    }

    private void startSocket() {

        String path="/data/data/com.example.socketserivce/cache/Image1";
        int totalSzie=0;//用来记录已传输文件的总大小
        byte buf[]=new byte[8192];
        int len;
        try {
            Socket socket=new Socket ("127.0.0.1",8888);
            Log.d (TAG, "成功连接到端口: ");
            File file=new File (path);
            //将文件夹中的文件添加到 列表中
            File[] files=file.listFiles ();

            if (file.exists ()){
                //socket中收到输出流
                DataOutputStream dout=new DataOutputStream (socket.getOutputStream ());
                //传入文件的个数
                dout.writeInt (files.length);
                //传入名字和大小
                for (int i=0;i<files.length;i++){
                    //在输出流中写入名字
                    dout.writeUTF (files[i].getName ());
                    dout.flush ();
                    //在输出流中写入文件大小
                    dout.writeLong (files[i].length ());
                    dout.flush ();
                    totalSzie+=files[i].length ();
                }

                dout.writeLong (totalSzie);
                Log.d (TAG, "传输文件总大小: "+totalSzie);
                for (int i=0;i<files.length;i++){
                    //缓存输入流
                    BufferedInputStream din=new BufferedInputStream (new FileInputStream (files[i]));
                    while((len=din.read (buf))!=-1){//-1表示为读取为空，din.read()返回读取的字节数,存储到一个字节数组buf中
                        dout.write (buf,0,len);//将一个字节数组中，从off开始,到len个字节写入到输出流
                    }
                    Log.d (TAG, "第几个文件: "+i);

                }
                Log.d (TAG, "发送完成。。。。: ");
                //关闭流
                dout.close ();
            }


        } catch (IOException e) {
            e.printStackTrace ();
        }
    }
}