背景

在日常开发和调试过程中，我们经常需要用到logcat的日志来帮助我们定位问题，在生产环境也有类似的需求，当程序出现崩溃、anr等异常时，我们除了需要捕获异常日志外，往往需要抓取一段完整的logcat日志，给我们研发同学去分析定位问题，但是生产环境是复杂的，如果当发生异常后，设备很有可能被重启了，这时候我们的logcat日志往往就丢失了，无法事后再去抓取，所以这时候就需要我们在程序中提供一种自动抓取logcat日志的能力。

logcat的小知识点

• 设备的logcat日志是有一个最大的缓存限制的，默认是256k，可以通过开发者选项去调整这个大小，也可以通过logcat -G 10M 设置。

• 设备重启后，logcat的缓存会被清空。

• 在终端中使用adb logcat命令获取log和在程序中调用logcat的命令获取log是两回事，获取到的日志信息往往是不同的，往往是在终端中获取的日志更全一些。

• 自定义ROM修改默认的logcat缓存大小

// kernel\drivers\staging\android\logger.c
// 在create_log方法的第二个参数传入自动定义的大小就好
static int __init

logger_init(void) {
    int ret;
    ret = create_log(LOGGER_LOG_MAIN, 256 * 1024 );
    if (unlikely(ret))
        goto out;

    ret = create_log(LOGGER_LOG_EVENTS, 256 * 1024);
    if (unlikely(ret))
        goto out;

    ret = create_log(LOGGER_LOG_RADIO, 256 * 1024 );
    if (unlikely(ret))
        goto out;

    ret = create_log(LOGGER_LOG_SYSTEM, 256 * 1024);
    if (unlikely(ret))
        goto out;

    out:
    return ret;
}

• 在程序中获取log需要加android.permission.READ_LOGS权限。

• android 4.1之前版本通过申请READ_LOGS权限就可以读取其他应用的log了。但是谷歌发现这样存在安全风险，于是android 4.1以及之后版本，即使申请了READ_LOGS权限也无法读取其他应用的日志信息了。4.1版本中 Logcat的签名变为 “signature|system|development”了，这意味着只有系统签名的app或者root权限的app才能使用该权限。普通用户可以通过ADB查看所有日志。

• 手机root后很多情况下也是无法在程序中使用root权限的，因为adb shell 的进程和程序执行的进程不是同一个，因此无法使用su命令。

抓取logcat的方式

• 方式一：读取日志流自己写入到文件中

private static boolean catchLogcatLog(String crashLogCatPath) {

        if(TextUtils.isEmpty(crashLogCatPath)){
            return false;
        }
        java.lang.Process logcatProcess = null;
        BufferedReader bufferedReader = null;
        BufferedOutputStream bufferedOutputStream = null;
        try {
            /** 获取系统logcat日志信息 */

            String[] running = new String[]{"logcat"};
            logcatProcess = Runtime.getRuntime().exec(running);

            bufferedReader = new BufferedReader(new InputStreamReader(
                    logcatProcess.getInputStream()));

            File file = new File(crashLogCatPath);
            File parentFile = file.getParentFile();
            if (!parentFile.exists() || !parentFile.isDirectory()) {
                parentFile.mkdirs();
            }
            bufferedOutputStream = new BufferedOutputStream(new FileOutputStream(file, true));

            String line;

            SimpleDateFormat simpleDateFormat = new SimpleDateFormat("MM-dd HH:mm:ss");
            String format = simpleDateFormat.format(new Date());
            byte[] lineBytes = "\n".getBytes("UTF-8");
            long s = SystemClock.elapsedRealtime();
            // 10s
            int maxWriteTime = 10000;
            while ((line = bufferedReader.readLine()) != null) {
                bufferedOutputStream.write(line.getBytes("UTF-8"));
                bufferedOutputStream.write(lineBytes);
                // 如果logcat打印到当前放生错误的位置或者打印时间大于10s就退出
                if (line.contains(format) || Math.abs((SystemClock.elapsedRealtime() - s)) > maxWriteTime) {
                    // 跳出打印
                    break;
                }
            }
            return true;

        } catch (Exception e) {
            Log.e("catch logcat error！");
        } finally {
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (bufferedOutputStream != null) {
                try {
                    bufferedOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return false;
    }

• 方式二：完全使用shell命令写文件

    private static boolean catchLogcatLogWithShell(String crashLogCatPath) {

        if (TextUtils.isEmpty(crashLogCatPath)) {
            return false;
        }
        ArrayList commnandList = new ArrayList();
        //如果不删除之前的logcat.txt文件，每次执行logcat命令也不会更新该文件
        commnandList.add("rm -r " + crashLogCatPath);
        // 使用-d参数可以让logcat获取日志完毕后终止进程
        // 如果使用">"是不会写入文件，必须使用-f的方式
        commnandList.add("logcat -d -v time -f " + crashLogCatPath);
        CommandResult commandResult = ShellUtils.execCommand(commnandList, false);
        return commandResult.result == 0;
    }

• 对比

方式一的实现逻辑更复杂，但是灵活性更好，可以在抓取的logcat文件中加入自定义信息，如在文件的开头写入设备的状态信息等。
方式二的实现比较简洁，完全交给系统处理文件的读写能比较好的保证日志的完整性。
大家可以根据自己的需求来选择实现方式。

工具类

• ShellUtils

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.List;

/**
 * ShellUtils
 * <ul>
 * <strong>Check root</strong>
 * <li>{@link ShellUtils#checkRootPermission()}</li>
 * </ul>
 * <ul>
 * <strong>Execte command</strong>
 * <li>{@link ShellUtils#execCommand(String, boolean)}</li>
 * <li>{@link ShellUtils#execCommand(String, boolean, boolean)}</li>
 * <li>{@link ShellUtils#execCommand(List, boolean)}</li>
 * <li>{@link ShellUtils#execCommand(List, boolean, boolean)}</li>
 * <li>{@link ShellUtils#execCommand(String[], boolean)}</li>
 * <li>{@link ShellUtils#execCommand(String[], boolean, boolean)}</li>
 * </ul>
 *
 */
public class ShellUtils {

    public static final String COMMAND_SU = "su";
    public static final String COMMAND_SH = "sh";
    public static final String COMMAND_EXIT = "exit\n";
    public static final String COMMAND_LINE_END = "\n";

    /**
     * check whether has root permission
     *
     * @return
     */
    public static boolean checkRootPermission() {

        return execCommand("echo root", true, false).result == 0;
    }

    /**
     * execute shell command
     *
     * @param command         command
     * @param isRoot          whether need to run with root
     * @param isNeedResultMsg whether need result msg
     *
     * @return
     *
     * @see ShellUtils#execCommand(String[], boolean, boolean)
     */
    public static CommandResult execCommand(String command, boolean isRoot, boolean isNeedResultMsg) {

        return execCommand(new String[]{command}, isRoot, isNeedResultMsg);
    }

    /**
     * execute shell commands
     *
     * @param commands        command array
     * @param isRoot          whether need to run with root
     * @param isNeedResultMsg whether need result msg
     *
     * @return <ul>
     * <li>if isNeedResultMsg is false, {@link CommandResult#successMsg} is null and
     * {@link CommandResult#errorMsg} is null.</li>
     * <li>if {@link CommandResult#result} is -1, there maybe some excepiton.</li>
     * </ul>
     */
    public static CommandResult execCommand(String[] commands, boolean isRoot, boolean isNeedResultMsg) {

        int result = -1;
        if (commands == null || commands.length == 0) {
            return new CommandResult(result, null, null);
        }

        Process process = null;
        BufferedReader successResult = null;
        BufferedReader errorResult = null;
        StringBuilder successMsg = null;
        StringBuilder errorMsg = null;

        DataOutputStream os = null;
        try {
            process = Runtime.getRuntime().exec(isRoot ? COMMAND_SU : COMMAND_SH);
            os = new DataOutputStream(process.getOutputStream());
            for (String command : commands) {
                if (command == null) {
                    continue;
                }

                // donnot use os.writeBytes(commmand), avoid chinese charset error
                os.write(command.getBytes());
                os.writeBytes(COMMAND_LINE_END);
                os.flush();
            }
            os.writeBytes(COMMAND_EXIT);
            os.flush();

            result = process.waitFor();
            // get command result
            if (isNeedResultMsg) {
                successMsg = new StringBuilder();
                errorMsg = new StringBuilder();
                successResult = new BufferedReader(new InputStreamReader(process.getInputStream()));
                errorResult = new BufferedReader(new InputStreamReader(process.getErrorStream()));
                String s;
                while ((s = successResult.readLine()) != null) {
                    successMsg.append(s);
                }
                while ((s = errorResult.readLine()) != null) {
                    errorMsg.append(s);
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (os != null) {
                    os.close();
                }
                if (successResult != null) {
                    successResult.close();
                }
                if (errorResult != null) {
                    errorResult.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }

            if (process != null) {
                process.destroy();
            }
        }
        return new CommandResult(result, successMsg == null ? null : successMsg.toString(), errorMsg == null ? null
                : errorMsg.toString());
    }

    /**
     * execute shell command, default return result msg
     *
     * @param command command
     * @param isRoot  whether need to run with root
     *
     * @return
     *
     * @see ShellUtils#execCommand(String[], boolean, boolean)
     */
    public static CommandResult execCommand(String command, boolean isRoot) {

        return execCommand(new String[]{command}, isRoot, true);
    }

    /**
     * execute shell commands, default return result msg
     *
     * @param commands command list
     * @param isRoot   whether need to run with root
     *
     * @return
     *
     * @see ShellUtils#execCommand(String[], boolean, boolean)
     */
    public static CommandResult execCommand(List<String> commands, boolean isRoot) {

        return execCommand(commands == null ? null : commands.toArray(new String[]{}), isRoot, true);
    }

    /**
     * execute shell commands, default return result msg
     *
     * @param commands command array
     * @param isRoot   whether need to run with root
     *
     * @return
     *
     * @see ShellUtils#execCommand(String[], boolean, boolean)
     */
    public static CommandResult execCommand(String[] commands, boolean isRoot) {

        return execCommand(commands, isRoot, true);
    }

    /**
     * execute shell commands
     *
     * @param commands        command list
     * @param isRoot          whether need to run with root
     * @param isNeedResultMsg whether need result msg
     *
     * @return
     *
     * @see ShellUtils#execCommand(String[], boolean, boolean)
     */
    public static CommandResult execCommand(List<String> commands, boolean isRoot, boolean isNeedResultMsg) {

        return execCommand(commands == null ? null : commands.toArray(new String[]{}), isRoot, isNeedResultMsg);
    }

    /**
     * result of command
     * <ul>
     * <li>{@link CommandResult#result} means result of command, 0 means normal, else means error, same to excute in
     * linux shell</li>
     * <li>{@link CommandResult#successMsg} means success message of command result</li>
     * <li>{@link CommandResult#errorMsg} means error message of command result</li>
     * </ul>
     *
     * @author <a href="http://www.trinea.cn" target="_blank">Trinea</a> 2013-5-16
     */
    public static class CommandResult {

        /**
         * result of command
         **/
        public int result;
        /**
         * success message of command result
         **/
        public String successMsg;
        /**
         * error message of command result
         **/
        public String errorMsg;

        public CommandResult(int result) {

            this.result = result;
        }

        public CommandResult(int result, String successMsg, String errorMsg) {

            this.result = result;
            this.successMsg = successMsg;
            this.errorMsg = errorMsg;
        }
    }
}