一、启动流程
DropBoxManagerService(简称DBMS) 记录着系统关键log信息，主要功能用于Debug调试。 Android系统启动过程SystemServer进程时，在startOtherServices()过程会启动DBMS服务，如下：

1.1 启动DBMS
[-> SystemServer.java]

private void startOtherServices() {
//初始化DBMS，并登记该服务【见小节1.2】
ServiceManager.addService(Context.DROPBOX_SERVICE,
new DropBoxManagerService(context, new File(“/data/system/dropbox”)));
…
}
其中DROPBOX_SERVICE = “dropbox”, DBMS工作目录位于”/data/system/dropbox”，这个过程向ServiceManager 登记名为“dropbox”的服务。那么可通过dumpsys dropbox来查看该dropbox服务信息。

1.2 初始化DBMS
[-> DropBoxManagerService.java]

public final class DropBoxManagerService extends IDropBoxManagerService.Stub {

public DropBoxManagerService(final Context context, File path) {    mDropBoxDir = path;  // 目录/data/system/dropbox    mContext = context;    mContentResolver = context.getContentResolver();    IntentFilter filter = new IntentFilter();    // 监听存储设备可用空间低的广播    filter.addAction(Intent.ACTION_DEVICE_STORAGE_LOW);    // 监听开机完毕的广播    filter.addAction(Intent.ACTION_BOOT_COMPLETED);    context.registerReceiver(mReceiver, filter);    // Settings数据库变化时则回调广播接收者的onReceive方法,此处CONTENT_URI=content://settings/global"    mContentResolver.registerContentObserver(        Settings.Global.CONTENT_URI, true,        new ContentObserver(new Handler()) {            public void onChange(boolean selfChange) {                mReceiver.onReceive(context, (Intent) null);            }        });    mHandler = new Handler() {        public void handleMessage(Message msg) {            // 发送广播            if (msg.what == MSG_SEND_BROADCAST) {                mContext.sendBroadcastAsUser((Intent)msg.obj, UserHandle.OWNER,                        android.Manifest.permission.READ_LOGS);            }        }    };}

}
该方法主要功能是给dropbox目录所对应的存储空间进行瘦身:

存储设备可用空间低；
开机完毕；
Settings数据库变化；
当发生任一以上情况都会触发触发执行mReceiver的onReceive方法,接下来看看该onReceive()过程.

1.3 mReceiver.onReceive
[-> DropBoxManagerService.java]

private final BroadcastReceiver mReceiver = new BroadcastReceiver() {
public void onReceive(Context context, Intent intent) {
if (intent != null && Intent.ACTION_BOOT_COMPLETED.equals(intent.getAction())) {
mBooted = true;
return;
}

    //收到ACTION_DEVICE_STORAGE_LOW，则强制重新check存储空间    mCachedQuotaUptimeMillis = 0;     //创建工作线程来执行init和trim操作    new Thread() {        public void run() {            try {                init(); //【见小节1.3.1】                trimToFit(); //【见小节1.3.2】            } catch (IOException e) {                ...            }        }    }.start();}

};
1.3.1 init
private synchronized void init() throws IOException {
if (mStatFs == null) {
if (!mDropBoxDir.isDirectory() && !mDropBoxDir.mkdirs()) {
…
}
mStatFs = new StatFs(mDropBoxDir.getPath());
mBlockSize = mStatFs.getBlockSize(); //mBlockSize=4096
}

if (mAllFiles == null) {    File[] files = mDropBoxDir.listFiles();    // 列举所有的dropbox文件    mAllFiles = new FileList();    mFilesByTag = new HashMap();    for (File file : files) {        if (file.getName().endsWith(".tmp")) {            file.delete(); //删除后缀为.tmp文件            continue;        }        // 创建dropbox的实体文件对象, 根据文件名来获取相应的时间戳        EntryFile entry = new EntryFile(file, mBlockSize);        if (entry.tag == null) {            continue; //忽略tag为空的文件        } else if (entry.timestampMillis == 0) {            file.delete(); //删除时间戳为0的文件            continue;        }        //将entry加入到mAllFiles对象        enrollEntry(entry);    }}

}
该方法主要功能：

创建目录/data/system/dropbox;
列举该目录下所有文件,并对其进行:
将每一个dropbox文件都对应于一个EntryFile对象,根据文件名来获取相应的时间戳
删除后缀为.tmp的文件;
删除时间戳为0的文件.
1.3.2 trimToFit
private synchronized long trimToFit() {

int ageSeconds = Settings.Global.getInt(mContentResolver,        Settings.Global.DROPBOX_AGE_SECONDS, DEFAULT_AGE_SECONDS);int maxFiles = Settings.Global.getInt(mContentResolver,        Settings.Global.DROPBOX_MAX_FILES, DEFAULT_MAX_FILES);long cutoffMillis = System.currentTimeMillis() - ageSeconds * 1000;while (!mAllFiles.contents.isEmpty()) {    EntryFile entry = mAllFiles.contents.first();    //当最老的文件时间戳在3天之内，且文件个数低于1000，则跳出循环    if (entry.timestampMillis > cutoffMillis         && mAllFiles.contents.size() < maxFiles) break;    FileList tag = mFilesByTag.get(entry.tag);    if (tag != null && tag.contents.remove(entry)) tag.blocks -= entry.blocks;    if (mAllFiles.contents.remove(entry)) mAllFiles.blocks -= entry.blocks;    if (entry.file != null) entry.file.delete(); //删除文件}long uptimeMillis = SystemClock.uptimeMillis();//除非接收设备存储低的广播，否则间隔5s才能再次执行restatif (uptimeMillis > mCachedQuotaUptimeMillis + QUOTA_RESCAN_MILLIS) {    int quotaPercent = Settings.Global.getInt(mContentResolver,            Settings.Global.DROPBOX_QUOTA_PERCENT, DEFAULT_QUOTA_PERCENT);    int reservePercent = Settings.Global.getInt(mContentResolver,            Settings.Global.DROPBOX_RESERVE_PERCENT, DEFAULT_RESERVE_PERCENT);    int quotaKb = Settings.Global.getInt(mContentResolver,            Settings.Global.DROPBOX_QUOTA_KB, DEFAULT_QUOTA_KB);    //重新统计文件    mStatFs.restat(mDropBoxDir.getPath());    int available = mStatFs.getAvailableBlocks();    int nonreserved = available - mStatFs.getBlockCount() * reservePercent / 100;    int maximum = quotaKb * 1024 / mBlockSize;    //可用的块数量    mCachedQuotaBlocks = Math.min(maximum, Math.max(0, nonreserved * quotaPercent / 100));    mCachedQuotaUptimeMillis = uptimeMillis;}if (mAllFiles.blocks > mCachedQuotaBlocks) {    //公平地限制所有tag的空间    int unsqueezed = mAllFiles.blocks, squeezed = 0;    TreeSet tags = new TreeSet(mFilesByTag.values());    for (FileList tag : tags) {        if (squeezed > 0 && tag.blocks <= (mCachedQuotaBlocks - unsqueezed) / squeezed) {            break;        }        unsqueezed -= tag.blocks;        squeezed++;    }    int tagQuota = (mCachedQuotaBlocks - unsqueezed) / squeezed;    //移除每个tags中的旧items    for (FileList tag : tags) {        if (mAllFiles.blocks < mCachedQuotaBlocks) break;        while (tag.blocks > tagQuota && !tag.contents.isEmpty()) {            EntryFile entry = tag.contents.first();            if (tag.contents.remove(entry)) tag.blocks -= entry.blocks;            if (mAllFiles.contents.remove(entry)) mAllFiles.blocks -= entry.blocks;            try {                if (entry.file != null) entry.file.delete();                enrollEntry(new EntryFile(mDropBoxDir, entry.tag, entry.timestampMillis));            } catch (IOException e) {                Slog.e(TAG, "Can't write tombstone file", e);            }        }    }}return mCachedQuotaBlocks * mBlockSize;

}
trimToFit过程中触发条件是：当文件有效时长超过3天，或者最大文件数超过1000，再或者剩余可用存储设备过低；

DBMS有很多常量参数：

DEFAULT_AGE_SECONDS = 3 * 86400：文件最长可存活时长为3天
DEFAULT_MAX_FILES = 1000：最大dropbox文件个数为1000
DEFAULT_QUOTA_KB = 5 * 1024：分配dropbox空间的最大值5M
DEFAULT_QUOTA_PERCENT = 10：是指dropbox目录最多可占用空间比例10%
DEFAULT_RESERVE_PERCENT = 10：是指dropbox不可使用的存储空间比例10%
QUOTA_RESCAN_MILLIS = 5000：重新扫描retrim时长为5s
当然上面这些都是默认值，完全可以通过设置content://settings/global数据库中相应项来设定值。

二、DropBox工作
当发生以下任一场景，都会调用AMS.addErrorToDropBox()来触发DBMS工作。

crash: 文章理解Android Crash处理流程 [小节4]的AMS.handleApplicationCrashInner过程
anr: 文章android ANR原理分析[小节3.1]的AMS.appNotResponding()过程；
watchdog: 文章WatchDog工作原理 [小节3.1]的Watchdog.run()过程;
native_crash: 当调用NativeCrashReporter.run()的过程;
wtf: 当调用Log.wtf()或者Log.wtfQuiet()的过程；
lowmem: 当内存较低时，触发AMS.reportMemUsage()过程；
…
2.1 AMS.addErrorToDropBox
[–>ActivityManagerService.java]

public void addErrorToDropBox(String eventType,
ProcessRecord process, String processName, ActivityRecord activity,
ActivityRecord parent, String subject,
final String report, final File logFile,
final ApplicationErrorReport.CrashInfo crashInfo) {

//创建dropbox标签名【见小节2.1.1】final String dropboxTag = processClass(process) + "_" + eventType;//获取dropbox服务的代理端final DropBoxManager dbox = (DropBoxManager)        mContext.getSystemService(Context.DROPBOX_SERVICE);//当不需要输出dropbox报告则直接返回if (dbox == null || !dbox.isTagEnabled(dropboxTag)) return;final StringBuilder sb = new StringBuilder(1024);//输出Process,flags,以及进程中所有package 【见小节2.1.2】appendDropBoxProcessHeaders(process, processName, sb);...if (subject != null) {    sb.append("Subject: ").append(subject).append("\n");}sb.append("Build: ").append(Build.FINGERPRINT).append("\n");sb.append("\n");//创建新线程，避免将调用者阻塞在I/OThread worker = new Thread("Error dump: " + dropboxTag) {    @Override    public void run() {        if (report != null) {            //比如ANR时输出Cpuinfo，或者lowmem时输出的内存信息            sb.append(report);         }        if (logFile != null) {            //比如anr或者Watchdog时输出的traces文件(kill -3)，最大上限为256KB            sb.append(FileUtils.readTextFile(logFile, DROPBOX_MAX_SIZE,                        "\n\n[[TRUNCATED]]"));        }        if (crashInfo != null && crashInfo.stackTrace != null) {            // 比如crash时输出的调用栈            sb.append(crashInfo.stackTrace);        }        String setting = Settings.Global.ERROR_LOGCAT_PREFIX + dropboxTag;        int lines = Settings.Global.getInt(mContext.getContentResolver(), setting, 0);        //当dropboxTag所对应的settings项不等于0，则输出logcat        if (lines > 0) {          //输出evets/system/main/crash这些log信息          java.lang.Process logcat = new ProcessBuilder("/system/bin/logcat",                  "-v", "time", "-b", "events", "-b", "system", "-b", "main",                  "-b", "crash",                  "-t", String.valueOf(lines)).redirectErrorStream(true).start();          input = new InputStreamReader(logcat.getInputStream());          int num;          char[] buf = new char[8192];          //不断读取input中的log内容，并添加到sb          while ((num = input.read(buf)) > 0) sb.append(buf, 0, num);          ...        }        //将log信息输出到DropBox 【见小节2.2】        dbox.addText(dropboxTag, sb.toString());    }};if (process == null) {    //当进程为空，意味着system_server进程崩溃，系统可能很快就要挂了,    //那么不再创建新线程，而是直接在system_server进程中同步运行    worker.run();} else {    //启动新线程    worker.start();}

}
该方法主要功能是输出以下内容项：

Process,flags, package等头信息；
当report不为空，则比如ANR时输出Cpuinfo，或者lowmem时输出的内存信息
当logFile不为空，则比如anr或者Watchdog时输出的traces文件(kill -3)，最大上限为256KB；
当stack不为空，则比如crash时输出的调用栈；
输出logcat的events/system/main/crash信息。
2.1.1 AMS.processClass
private static String processClass(ProcessRecord process) {
//MY_PID代表的是当前进程pid，正是system_server进程
if (process == null || process.pid == MY_PID) {
return “system_server”;
} else if ((process.info.flags & ApplicationInfo.FLAG_SYSTEM) != 0) {
return “system_app”;
} else {
return “data_app”;
}
}
dropbox文件名格式为dropboxTag@xxx.txt xxx代表时间戳,例如system_server_crash@1465650845355.txt,则记录该文件时间戳为1465650845355. 文件后缀除了.txt，还有压缩格式.txt.gz. 对于dropboxTag是由processClass + eventType组合而成.

processClass分为system_server, system_app, data_app;
eventType：分为crash,anr,wtf,native_cras,lowmem, watchdog
列举部分常见tags以及含义:

dropboxTag 含义
system_server_anr system进程无响应
system_server_watchdog system进程发生watchdog
system_server_crash system进程崩溃
system_server_native_crash system进程native出现崩溃
system_server_wtf system进程发生严重错误
system_server_lowmem system进程内存不足
当然除了system_server进程, 还有system_app, data_app类型的进程, 以上所有类型都适用,列举部分:

system_app_crash 系统app崩溃
system_app_anr 系统app无响应
data_app_crash 普通app崩溃
data_app_anr 普通app无响应
2.1.2 AMS.appendDropBoxProcessHeaders
private void appendDropBoxProcessHeaders(ProcessRecord process, String processName,
StringBuilder sb) {
if (process == null) {
sb.append(“Process: “).append(processName).append(“\n”);
return;
}

synchronized (this) {    sb.append("Process: ").append(processName).append("\n");    int flags = process.info.flags;    IPackageManager pm = AppGlobals.getPackageManager();    sb.append("Flags: 0x").append(Integer.toString(flags, 16)).append("\n");    for (int ip=0; ip

    int read = 0;    while (read < buffer.length) {        int n = input.read(buffer, read, buffer.length - read);        if (n <= 0) break;        read += n;    }    //创建临时文件，例如tid=1234的对应文件drop1234.tmp    temp = new File(mDropBoxDir, "drop" + Thread.currentThread().getId() + ".tmp");    int bufferSize = mBlockSize;    if (bufferSize > 4096) bufferSize = 4096;    if (bufferSize < 512) bufferSize = 512;    FileOutputStream foutput = new FileOutputStream(temp);    output = new BufferedOutputStream(foutput, bufferSize);    //创建gzip压缩文件    if (read == buffer.length && ((flags & DropBoxManager.IS_GZIPPED) == 0)) {        output = new GZIPOutputStream(output);        flags = flags | DropBoxManager.IS_GZIPPED;    }    //不断将temp文件数据写入buffer    do {        output.write(buffer, 0, read);        long now = System.currentTimeMillis();        if (now - lastTrim > 30 * 1000) {            max = trimToFit(); //执行时间超过30s则执行trim            lastTrim = now;        }        read = input.read(buffer);        if (read <= 0) {            FileUtils.sync(foutput);            output.close();            output = null;        } else {            output.flush();        }        long len = temp.length();        if (len > max) {            temp.delete();            temp = null;            break;        }    } while (read > 0);    //[见小节2.3.1]    long time = createEntry(temp, tag, flags);    temp = null;    final Intent dropboxIntent = new Intent(DropBoxManager.ACTION_DROPBOX_ENTRY_ADDED);    dropboxIntent.putExtra(DropBoxManager.EXTRA_TAG, tag);    dropboxIntent.putExtra(DropBoxManager.EXTRA_TIME, time);    if (!mBooted) {        dropboxIntent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);    }    //发送广播MSG_SEND_BROADCAST    mHandler.sendMessage(mHandler.obtainMessage(MSG_SEND_BROADCAST, dropboxIntent));} catch (IOException e) {    ...} finally {    if (output != null) output.close();    entry.close();    if (temp != null) temp.delete();}