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Android是建立在Linux之上的OS，在涉及到安全、网络协议、文件加密等功能时，往往需要通过C语言调用底层API来实现，而如何发出指令让C端执行我们想要的功能，并且在执行之后有返回结果呢，这就需要打通Java端进程和C端进程，使之能高效地通信。这样，C端进程用于实现功能，Java端进程负责UI、功能的触发及结果处理就可以了。
对于*nix系统来说，“一切皆为文件”，Socket也不例外，Socket按照收发双方的媒介来说有三种类型：1，通过网络端口；2，通过文件系统；3，通过内存映射文件。具体说来，三种类型均可以用来作为IPC的Socket：1，通过本地回环接口(即LoopBack)127.0.0.1来收发数据；2，通过文件作为收发数据的中转站；3，在内存中开辟一块区域作为收发数据的中转站，此区域仍然使用文件读写API进行访问。LocalSocket支持方式2和方式3，从效率的角度来说，显然是方式3效率最高，那么下面我们就使用LocalSocket来演示如何实现Java端进程与C端进程之间的IPC。
以下的demo是Java端作为server，C端作为client；实际场景中可能更多的是Java端作为client，而C端作为server。
服务端代码如下：
1 package main.activity; 2 3 import java.io.IOException; 4 import java.io.InputStream; 5 import java.io.InputStreamReader; 6 7 import android.app.Activity; 8 import android.net.LocalServerSocket; 9 import android.net.LocalSocket; 10 import android.os.Bundle; 11 import android.util.Log; 12 13 /** 14 * @author pengyiming 15 * @note 启动localSocketServer 16 * 17 */ 18 19 public class LocalSocketServerActivity extends Activity 20 { 21 /* 数据段begin */ 22 private final String TAG = "server"; 23 24 private ServerSocketThread mServerSocketThread; 25 /* 数据段end */ 26 27 /* 函数段begin */ 28 @Override 29 protected void onCreate(Bundle savedInstanceState) 30 { 31 super.onCreate(savedInstanceState); 32 33 mServerSocketThread = new ServerSocketThread(); 34 mServerSocketThread.start(); 35 } 36 37 @Override 38 protected void onDestroy() 39 { 40 super.onDestroy(); 41 42 mServerSocketThread.stopRun(); 43 } 44 /* 函数段end */ 45 46 /* 内部类begin */ 47 private class ServerSocketThread extends Thread 48 { 49 private boolean keepRunning = true; 50 private LocalServerSocket serverSocket; 51 52 private void stopRun() 53 { 54 keepRunning = false; 55 } 56 57 @Override 58 public void run() 59 { 60 try 61 { 62 serverSocket = new LocalServerSocket("pym_local_socket"); 63 } 64 catch (IOException e) 65 { 66 e.printStackTrace(); 67 68 keepRunning = false; 69 } 70 71 while(keepRunning) 72 { 73 Log.d(TAG, "wait for new client coming !"); 74 75 try 76 { 77 LocalSocket interactClientSocket = serverSocket.accept(); 78 79 //由于accept()在阻塞时，可能Activity已经finish掉了，所以再次检查keepRunning 80 if (keepRunning) 81 { 82 Log.d(TAG, "new client coming !"); 83 84 new InteractClientSocketThread(interactClientSocket).start(); 85 } 86 } 87 catch (IOException e) 88 { 89 e.printStackTrace(); 90 91 keepRunning = false; 92 } 93 } 94 95 if (serverSocket != null) 96 { 97 try 98 { 99 serverSocket.close();100 }101 catch (IOException e)102 {103 e.printStackTrace();104 }105 }106 }107 }108 109 private class InteractClientSocketThread extends Thread110 {111 private LocalSocket interactClientSocket;112 113 public InteractClientSocketThread(LocalSocket interactClientSocket)114 {115 this.interactClientSocket = interactClientSocket;116 }117 118 @Override119 public void run()120 {121 StringBuilder recvStrBuilder = new StringBuilder();122 InputStream inputStream = null;123 try124 {125 inputStream = interactClientSocket.getInputStream();126 InputStreamReader inputStreamReader = new InputStreamReader(inputStream);127 char[] buf = new char[4096];128 int readBytes = -1;129 while ((readBytes = inputStreamReader.read(buf)) != -1)130 {131 String tempStr = new String(buf, 0, readBytes);132 recvStrBuilder.append(tempStr);133 }134 }135 catch (IOException e)136 {137 e.printStackTrace();138 139 Log.d(TAG, "resolve data error !");140 }141 finally142 {143 if (inputStream != null)144 {145 try146 {147 inputStream.close();148 }149 catch (IOException e)150 {151 e.printStackTrace();152 }153 }154 }155 }156 }157 /* 内部类end */158 } 客户端代码如下：
1 package main.activity; 2 3 import android.app.Activity; 4 import android.os.Bundle; 5 import android.util.Log; 6 7 /** 8 * @author pengyiming 9 * @note 用于启动localSocketClient，向server发送心跳报文10 *11 */12 13 public class LocalSocketClientActivity extends Activity14 {15 /* 数据段begin */16 private final String TAG = "client";17 18 private HeartBeatThread mHeartBeatThread;19 20 public native int startHeartBeat();21 /* 数据段end */22 23 /* 函数段begin */24 static25 {26 System.loadLibrary("pymclient");27 }28 29 @Override30 protected void onCreate(Bundle savedInstanceState)31 {32 super.onCreate(savedInstanceState);33 34 mHeartBeatThread = new HeartBeatThread();35 mHeartBeatThread.start();36 }37 38 @Override39 protected void onDestroy()40 {41 super.onDestroy();42 43 mHeartBeatThread.stopRun();44 }45 /* 函数段end */46 47 /* 内部类begin */48 private class HeartBeatThread extends Thread49 {50 int ret;51 boolean keepRunning = true;52 53 public void stopRun()54 {55 keepRunning = false;56 }57 58 @Override59 public void run()60 {61 Log.d(TAG, "start heart beat!");62 63 while (keepRunning)64 {65 ret = startHeartBeat();66 67 Log.d(TAG, "ret = " + ret);68 69 if (ret != 0)70 {71 break;72 }73 74 try75 {76 sleep(1000);77 }78 catch (InterruptedException e)79 {80 e.printStackTrace();81 }82 }83 84 Log.d(TAG, "stop heart beat!");85 }86 }87 /* 内部类end */88 } 上述客户端代码启动了一个线程用于发送“心跳”报文，每隔1s构建一个新的LocalSocket，连接服务端并发送流数据，其核心就在于native方法的实现。值得一提的是，我最初使用原生socket函数，没想connect总是返回错误；后来在同事的提醒下，我参考了Android源码rild.c中socket_local_client的使用，并从socket_local_client.c中抽取出相应代码改写而成。
客户端native方法头文件：
1 /* DO NOT EDIT THIS FILE - it is machine generated */ 2 #include3 /* Header for class main_activity_LocalSocketClientActivity */ 4 5 #ifndef _Included_main_activity_LocalSocketClientActivity 6 #define _Included_main_activity_LocalSocketClientActivity 7 #ifdef __cplusplus 8 extern "C" { 9 #endif10 11 /* socket命名空间(见cutils/sockets.h) */12 #define ANDROID_SOCKET_NAMESPACE_ABSTRACT 013 #define ANDROID_SOCKET_NAMESPACE_RESERVED 114 #define ANDROID_SOCKET_NAMESPACE_FILESYSTEM 215 16 /* socket类型 */17 #define SOCK_STREAM 118 #define SOCK_DGRAM 219 #define SOCK_RAW 320 #define SOCK_RDM 421 #define SOCK_SEQPACKET 522 #define SOCK_PACKET 1023 24 /* 清0宏 */25 #define MEM_ZERO(pDest, destSize) memset(pDest, 0, destSize)26 27 /* 错误码定义 */28 #define NO_ERR 029 #define CREATE_ERR -130 #define CONNECT_ERR -231 #define LINUX_MAKE_ADDRUN_ERROR -332 #define NO_LINUX_MAKE_ADDRUN_ERROR -433 #define CLOSE_ERR -534 35 /* 是否使用linux的本地socket命令空间 */36 #define HAVE_LINUX_LOCAL_SOCKET_NAMESPACE "linux_local_socket_namespace"37 38 #undef main_activity_LocalSocketClientActivity_MODE_PRIVATE39 #define main_activity_LocalSocketClientActivity_MODE_PRIVATE 0L40 #undef main_activity_LocalSocketClientActivity_MODE_WORLD_READABLE41 #define main_activity_LocalSocketClientActivity_MODE_WORLD_READABLE 1L42 #undef main_activity_LocalSocketClientActivity_MODE_WORLD_WRITEABLE43 #define main_activity_LocalSocketClientActivity_MODE_WORLD_WRITEABLE 2L44 #undef main_activity_LocalSocketClientActivity_MODE_APPEND45 #define main_activity_LocalSocketClientActivity_MODE_APPEND 32768L46 #undef main_activity_LocalSocketClientActivity_MODE_MULTI_PROCESS47 #define main_activity_LocalSocketClientActivity_MODE_MULTI_PROCESS 4L48 #undef main_activity_LocalSocketClientActivity_BIND_AUTO_CREATE49 #define main_activity_LocalSocketClientActivity_BIND_AUTO_CREATE 1L50 #undef main_activity_LocalSocketClientActivity_BIND_DEBUG_UNBIND51 #define main_activity_LocalSocketClientActivity_BIND_DEBUG_UNBIND 2L52 #undef main_activity_LocalSocketClientActivity_BIND_NOT_FOREGROUND53 #define main_activity_LocalSocketClientActivity_BIND_NOT_FOREGROUND 4L54 #undef main_activity_LocalSocketClientActivity_BIND_ABOVE_CLIENT55 #define main_activity_LocalSocketClientActivity_BIND_ABOVE_CLIENT 8L56 #undef main_activity_LocalSocketClientActivity_BIND_ALLOW_OOM_MANAGEMENT57 #define main_activity_LocalSocketClientActivity_BIND_ALLOW_OOM_MANAGEMENT 16L58 #undef main_activity_LocalSocketClientActivity_BIND_WAIVE_PRIORITY59 #define main_activity_LocalSocketClientActivity_BIND_WAIVE_PRIORITY 32L60 #undef main_activity_LocalSocketClientActivity_BIND_IMPORTANT61 #define main_activity_LocalSocketClientActivity_BIND_IMPORTANT 64L62 #undef main_activity_LocalSocketClientActivity_BIND_ADJUST_WITH_ACTIVITY63 #define main_activity_LocalSocketClientActivity_BIND_ADJUST_WITH_ACTIVITY 128L64 #undef main_activity_LocalSocketClientActivity_CONTEXT_INCLUDE_CODE65 #define main_activity_LocalSocketClientActivity_CONTEXT_INCLUDE_CODE 1L66 #undef main_activity_LocalSocketClientActivity_CONTEXT_IGNORE_SECURITY67 #define main_activity_LocalSocketClientActivity_CONTEXT_IGNORE_SECURITY 2L68 #undef main_activity_LocalSocketClientActivity_CONTEXT_RESTRICTED69 #define main_activity_LocalSocketClientActivity_CONTEXT_RESTRICTED 4L70 #undef main_activity_LocalSocketClientActivity_RESULT_CANCELED71 #define main_activity_LocalSocketClientActivity_RESULT_CANCELED 0L72 #undef main_activity_LocalSocketClientActivity_RESULT_OK73 #define main_activity_LocalSocketClientActivity_RESULT_OK -1L74 #undef main_activity_LocalSocketClientActivity_RESULT_FIRST_USER75 #define main_activity_LocalSocketClientActivity_RESULT_FIRST_USER 1L76 #undef main_activity_LocalSocketClientActivity_DEFAULT_KEYS_DISABLE77 #define main_activity_LocalSocketClientActivity_DEFAULT_KEYS_DISABLE 0L78 #undef main_activity_LocalSocketClientActivity_DEFAULT_KEYS_DIALER79 #define main_activity_LocalSocketClientActivity_DEFAULT_KEYS_DIALER 1L80 #undef main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SHORTCUT81 #define main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SHORTCUT 2L82 #undef main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SEARCH_LOCAL83 #define main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SEARCH_LOCAL 3L84 #undef main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SEARCH_GLOBAL85 #define main_activity_LocalSocketClientActivity_DEFAULT_KEYS_SEARCH_GLOBAL 4L86 /*87 * Class: main_activity_LocalSocketClientActivity88 * Method: startHeartBeat89 * Signature: ()I90 */91 JNIEXPORT jint JNICALL Java_main_activity_LocalSocketClientActivity_startHeartBeat92 (JNIEnv *, jobject);93 94 #ifdef __cplusplus95 }96 #endif97 #endif
客户端native方法实现：
1 /* 头文件begin */ 2 #include "main_activity_LocalSocketClientActivity.h" 3 4 #include5 #include 6 #include 7 #include 8 /* 头文件end */ 9 10 #ifdef __cplusplus 11 extern "C" { 12 #endif 13 14 /* 15 * Class: main_activity_LocalSocketClientActivity 16 * Method: startHeartBeat 17 */ 18 JNIEXPORT jint JNICALL Java_main_activity_LocalSocketClientActivity_startHeartBeat(JNIEnv * env, jobject object) 19 { 20 int socketID; 21 struct sockaddr_un serverAddr; 22 char path[] = "pym_local_socket\0"; 23 int ret; 24 25 socketID = socket_local_client(path, ANDROID_SOCKET_NAMESPACE_ABSTRACT, SOCK_STREAM); 26 if (socketID < 0) 27 { 28 return socketID; 29 } 30 31 ret = close(socketID); 32 if (ret < 0) 33 { 34 return CLOSE_ERR; 35 } 36 37 return NO_ERR; 38 } 39 40 /* 创建本地socket客户端 */ 41 int socket_local_client(const char *name, int namespaceId, int type) 42 { 43 int socketID; 44 int ret; 45 46 socketID = socket(AF_LOCAL, type, 0); 47 if(socketID < 0) 48 { 49 return CREATE_ERR; 50 } 51 52 ret = socket_local_client_connect(socketID, name, namespaceId, type); 53 if (ret < 0) 54 { 55 close(socketID); 56 57 return ret; 58 } 59 60 return socketID; 61 } 62 63 /* 连接到相应的fileDescriptor上 */ 64 int socket_local_client_connect(int fd, const char *name, int namespaceId, int type) 65 { 66 struct sockaddr_un addr; 67 socklen_t socklen; 68 size_t namelen; 69 int ret; 70 71 ret = socket_make_sockaddr_un(name, namespaceId, &addr, &socklen); 72 if (ret < 0) 73 { 74 return ret; 75 } 76 77 if(connect(fd, (struct sockaddr *) &addr, socklen) < 0) 78 { 79 return CONNECT_ERR; 80 } 81 82 return fd; 83 } 84 85 /* 构造sockaddr_un */ 86 int socket_make_sockaddr_un(const char *name, int namespaceId, struct sockaddr_un *p_addr, socklen_t *socklen) 87 { 88 size_t namelen; 89 90 MEM_ZERO(p_addr, sizeof(*p_addr)); 91 #ifdef HAVE_LINUX_LOCAL_SOCKET_NAMESPACE 92 93 namelen = strlen(name); 94 95 // Test with length +1 for the *initial* '\0'. 96 if ((namelen + 1) > sizeof(p_addr->sun_path)) 97 { 98 return LINUX_MAKE_ADDRUN_ERROR; 99 }100 p_addr->sun_path[0] = 0;101 memcpy(p_addr->sun_path + 1, name, namelen);102 103 #else104 105 namelen = strlen(name) + strlen(FILESYSTEM_SOCKET_PREFIX);106 107 /* unix_path_max appears to be missing on linux */108 if (namelen > (sizeof(*p_addr) - offsetof(struct sockaddr_un, sun_path) - 1))109 {110 return NO_LINUX_MAKE_ADDRUN_ERROR;111 }112 113 strcpy(p_addr->sun_path, FILESYSTEM_SOCKET_PREFIX);114 strcat(p_addr->sun_path, name);115 116 #endif117 118 p_addr->sun_family = AF_LOCAL;119 *socklen = namelen + offsetof(struct sockaddr_un, sun_path) + 1;120 121 return NO_ERR;122 }123 124 #ifdef __cplusplus125 }126 #endif
注意到100~101行比较特殊，是从p_addr->sun_path[1]开始拷贝本地域名，这就是之前为什么一直connect不上的原因，至于为什么偏移1个字节来拷贝本地域名，你可以在*nix系统下输入"man 7 unix"来找到原因。
先启动server，再启动client就可以看到结果了。对了，在成功创建并已自动连接后，我并未发送任何数据，其实发送数据就是写入文件，It's your trun now! 在close之前加入这段代码吧~
1 int ret;2 char buf[] = "hello";3 4 ret = write(socketID, buf, strlen(buf));
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