将 android 客户端与 linux 中的 c 服务器连接

Question

I have to create a simple android application that communicates with a server in c using only the java .net library. I wrote this server in c on a linux virtual machine and the application using android studio on windows but it seems that this does not connect to the server

Client.java

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

import androidx.appcompat.app.AppCompatActivity;

import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;

public class MainActivity extends AppCompatActivity {

    private Socket socket;

    private static final int SERVERPORT = 8888;
    private static final String SERVER_IP = "10.0.2.15";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new ClientThread()).start();
    }

    public void onClick(View view) {
        try {
            EditText et = findViewById(R.id.EditText01);
            String str = et.getText().toString();
            PrintWriter out = new PrintWriter(new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())),
                    true);
            out.println(str);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    class ClientThread implements Runnable {

        @Override
        public void run() {

            try {
                InetAddress serverAddr = InetAddress.getByName(SERVER_IP);

                socket = new Socket(serverAddr, SERVERPORT);

            } catch (IOException e1) {
                e1.printStackTrace();
            }

        }

    }
}

server.c

/*
    C socket server example
*/

#include<stdio.h>
#include<string.h>    //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<unistd.h>    //write

int main(int argc , char *argv[])
{
    int socket_desc , client_sock , c , read_size;
    struct sockaddr_in server , client;
    char client_message[2000];

    //Create socket
    socket_desc = socket(AF_INET , SOCK_STREAM , 0);
    if (socket_desc == -1)
    {
        printf("Could not create socket");
    }
    puts("Socket created");

    //Prepare the sockaddr_in structure
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons( 8888 );

    //Bind
    if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
    {
        //print the error message
        perror("bind failed. Error");
        return 1;
    }
    puts("bind done");

    //Listen
    listen(socket_desc , 3);

    //Accept and incoming connection
    puts("Waiting for incoming connections...");
    c = sizeof(struct sockaddr_in);

    //accept connection from an incoming client
    client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
    if (client_sock < 0)
    {
        perror("accept failed");
        return 1;
    }
    puts("Connection accepted");

    //Receive a message from client
    while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
    {
        //Send the message back to client
        write(client_sock , client_message , strlen(client_message));
    }

    if(read_size == 0)
    {
        puts("Client disconnected");
        fflush(stdout);
    }
    else if(read_size == -1)
    {
        perror("recv failed");
    }

    return 0;
}

This is the logcat when I try to send the message by pressing the button:

2021-04-27 08:34:40.858 1185-1185/com.example.androdi_socket_example W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.OutputStream java.net.Socket.getOutputStream()' on a null object reference
2021-04-27 08:34:40.859 1185-1185/com.example.androdi_socket_example W/System.err:     at com.example.androdi_socket_example.MainActivity.onClick(MainActivity.java:36)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.view.View.performClick(View.java:6294)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1119)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.view.View$PerformClick.run(View.java:24770)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Handler.handleCallback(Handler.java:790)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:99)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Looper.loop(Looper.java:164)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6494)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

I have to create a simple android application that communicates with a server in c using only the java .net library. I wrote this server in c on a linux virtual machine and the application using android studio on windows but it seems that this does not connect to the server

Client.java

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

import androidx.appcompat.app.AppCompatActivity;

import java.io.BufferedWriter;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;

public class MainActivity extends AppCompatActivity {

    private Socket socket;

    private static final int SERVERPORT = 8888;
    private static final String SERVER_IP = "10.0.2.15";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        new Thread(new ClientThread()).start();
    }

    public void onClick(View view) {
        try {
            EditText et = findViewById(R.id.EditText01);
            String str = et.getText().toString();
            PrintWriter out = new PrintWriter(new BufferedWriter(
                    new OutputStreamWriter(socket.getOutputStream())),
                    true);
            out.println(str);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    class ClientThread implements Runnable {

        @Override
        public void run() {

            try {
                InetAddress serverAddr = InetAddress.getByName(SERVER_IP);

                socket = new Socket(serverAddr, SERVERPORT);

            } catch (IOException e1) {
                e1.printStackTrace();
            }

        }

    }
}

server.c

/*
    C socket server example
*/

#include<stdio.h>
#include<string.h>    //strlen
#include<sys/socket.h>
#include<arpa/inet.h> //inet_addr
#include<unistd.h>    //write

int main(int argc , char *argv[])
{
    int socket_desc , client_sock , c , read_size;
    struct sockaddr_in server , client;
    char client_message[2000];

    //Create socket
    socket_desc = socket(AF_INET , SOCK_STREAM , 0);
    if (socket_desc == -1)
    {
        printf("Could not create socket");
    }
    puts("Socket created");

    //Prepare the sockaddr_in structure
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons( 8888 );

    //Bind
    if( bind(socket_desc,(struct sockaddr *)&server , sizeof(server)) < 0)
    {
        //print the error message
        perror("bind failed. Error");
        return 1;
    }
    puts("bind done");

    //Listen
    listen(socket_desc , 3);

    //Accept and incoming connection
    puts("Waiting for incoming connections...");
    c = sizeof(struct sockaddr_in);

    //accept connection from an incoming client
    client_sock = accept(socket_desc, (struct sockaddr *)&client, (socklen_t*)&c);
    if (client_sock < 0)
    {
        perror("accept failed");
        return 1;
    }
    puts("Connection accepted");

    //Receive a message from client
    while( (read_size = recv(client_sock , client_message , 2000 , 0)) > 0 )
    {
        //Send the message back to client
        write(client_sock , client_message , strlen(client_message));
    }

    if(read_size == 0)
    {
        puts("Client disconnected");
        fflush(stdout);
    }
    else if(read_size == -1)
    {
        perror("recv failed");
    }

    return 0;
}

This is the logcat when I try to send the message by pressing the button:

2021-04-27 08:34:40.858 1185-1185/com.example.androdi_socket_example W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.OutputStream java.net.Socket.getOutputStream()' on a null object reference
2021-04-27 08:34:40.859 1185-1185/com.example.androdi_socket_example W/System.err:     at com.example.androdi_socket_example.MainActivity.onClick(MainActivity.java:36)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:409)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.view.View.performClick(View.java:6294)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1119)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.view.View$PerformClick.run(View.java:24770)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Handler.handleCallback(Handler.java:790)
2021-04-27 08:34:40.860 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Handler.dispatchMessage(Handler.java:99)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at android.os.Looper.loop(Looper.java:164)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at android.app.ActivityThread.main(ActivityThread.java:6494)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at java.lang.reflect.Method.invoke(Native Method)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
2021-04-27 08:34:40.861 1185-1185/com.example.androdi_socket_example W/System.err:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)