Android 源码分析（七） Launcher 桌面程序启动分析

//ZygoteServer.java
 void runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller {
        ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
        ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();

        fds.add(mServerSocket.getFileDescriptor());
        peers.add(null);

        while (true) {
            StructPollfd[] pollFds = new StructPollfd[fds.size()];
            for (int i = 0; i < pollFds.length; ++i) {
                pollFds[i] = new StructPollfd();
                pollFds[i].fd = fds.get(i);
                pollFds[i].events = (short) POLLIN;
            }
            try {
                Os.poll(pollFds, -1);
            } catch (ErrnoException ex) {
                throw new RuntimeException("poll failed", ex);
            }
            for (int i = pollFds.length - 1; i >= 0; --i) {
                if ((pollFds[i].revents & POLLIN) == 0) {
                    continue;
                }
                if (i == 0) {
                    ZygoteConnection newPeer = acceptCommandPeer(abiList);
                    peers.add(newPeer);
                    fds.add(newPeer.getFileDesciptor());
                } else {
                    boolean done = peers.get(i).runOnce(this);
                    if (done) {
                        peers.remove(i);
                        fds.remove(i);
                    }
                }
            }
        }
    }
}

　　/**
     * Waits for and accepts a single command connection. Throws
     * RuntimeException on failure.
     */
    private ZygoteConnection acceptCommandPeer(String abiList) {
        try {
            return createNewConnection(mServerSocket.accept(), abiList);
        } catch (IOException ex) {
            throw new RuntimeException(
                    "IOException during accept()", ex);
        }
    }
 
 protected ZygoteConnection createNewConnection(LocalSocket socket, String abiList)
            throws IOException {
        return new ZygoteConnection(socket, abiList);
    }

//ZygoteConnection.java
    /**
     * Constructs instance from connected socket.
     *
     * @param socket non-null; connected socket
     * @param abiList non-null; a list of ABIs this zygote supports.
     * @throws IOException
     */
    ZygoteConnection(LocalSocket socket, String abiList) throws IOException {
        mSocket = socket;
        this.abiList = abiList;

        mSocketOutStream
                = new DataOutputStream(socket.getOutputStream());

        mSocketReader = new BufferedReader(
                new InputStreamReader(socket.getInputStream()), 256);

        mSocket.setSoTimeout(CONNECTION_TIMEOUT_MILLIS);

        try {
            peer = mSocket.getPeerCredentials();
        } catch (IOException ex) {
            Log.e(TAG, "Cannot read peer credentials", ex);
            throw ex;
        }
    }
    
    /**
     * Reads one start command from the command socket. If successful,
     * a child is forked and a {@link Zygote.MethodAndArgsCaller}
     * exception is thrown in that child while in the parent process,
     * the method returns normally. On failure, the child is not
     * spawned and messages are printed to the log and stderr. Returns
     * a boolean status value indicating whether an end-of-file on the command
     * socket has been encountered.
     *
     * @return false if command socket should continue to be read from, or
     * true if an end-of-file has been encountered.
     * @throws Zygote.MethodAndArgsCaller trampoline to invoke main()
     * method in child process
     */
    boolean runOnce(ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller {

        String args[];
        Arguments parsedArgs = null;
        FileDescriptor[] descriptors;

        try {
            args = readArgumentList();
            descriptors = mSocket.getAncillaryFileDescriptors();
        } catch (IOException ex) {
            Log.w(TAG, "IOException on command socket " + ex.getMessage());
            closeSocket();
            return true;
        }

        if (args == null) {
            // EOF reached.
            closeSocket();
            return true;
        }

        /** the stderr of the most recent request, if avail */
        PrintStream newStderr = null;

        if (descriptors != null && descriptors.length >= 3) {
            newStderr = new PrintStream(
                    new FileOutputStream(descriptors[2]));
        }

        int pid = -1;
        FileDescriptor childPipeFd = null;
        FileDescriptor serverPipeFd = null;

        try {
            parsedArgs = new Arguments(args);

            if (parsedArgs.abiListQuery) {
                return handleAbiListQuery();
            }

            if (parsedArgs.preloadDefault) {
                return handlePreload();
            }

            if (parsedArgs.preloadPackage != null) {
                return handlePreloadPackage(parsedArgs.preloadPackage,
                        parsedArgs.preloadPackageLibs, parsedArgs.preloadPackageCacheKey);
            }

            if (parsedArgs.permittedCapabilities != 0 || parsedArgs.effectiveCapabilities != 0) {
                throw new ZygoteSecurityException("Client may not specify capabilities: " +
                        "permitted=0x" + Long.toHexString(parsedArgs.permittedCapabilities) +
                        ", effective=0x" + Long.toHexString(parsedArgs.effectiveCapabilities));
            }

            applyUidSecurityPolicy(parsedArgs, peer);
            applyInvokeWithSecurityPolicy(parsedArgs, peer);

            applyDebuggerSystemProperty(parsedArgs);
            applyInvokeWithSystemProperty(parsedArgs);

            int[][] rlimits = null;

            if (parsedArgs.rlimits != null) {
                rlimits = parsedArgs.rlimits.toArray(intArray2d);
            }

            int[] fdsToIgnore = null;

            if (parsedArgs.invokeWith != null) {
                FileDescriptor[] pipeFds = Os.pipe2(O_CLOEXEC);
                childPipeFd = pipeFds[1];
                serverPipeFd = pipeFds[0];
                Os.fcntlInt(childPipeFd, F_SETFD, 0);
                fdsToIgnore = new int[] { childPipeFd.getInt$(), serverPipeFd.getInt$() };
            }

            /**
             * In order to avoid leaking descriptors to the Zygote child,
             * the native code must close the two Zygote socket descriptors
             * in the child process before it switches from Zygote-root to
             * the UID and privileges of the application being launched.
             *
             * In order to avoid "bad file descriptor" errors when the
             * two LocalSocket objects are closed, the Posix file
             * descriptors are released via a dup2() call which closes
             * the socket and substitutes an open descriptor to /dev/null.
             */

            int [] fdsToClose = { -1, -1 };

            FileDescriptor fd = mSocket.getFileDescriptor();

            if (fd != null) {
                fdsToClose[0] = fd.getInt$();
            }

            fd = zygoteServer.getServerSocketFileDescriptor();

            if (fd != null) {
                fdsToClose[1] = fd.getInt$();
            }

            fd = null;

            pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
                    parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
                    parsedArgs.niceName, fdsToClose, fdsToIgnore, parsedArgs.instructionSet,
                    parsedArgs.appDataDir);
        } catch (ErrnoException ex) {
            logAndPrintError(newStderr, "Exception creating pipe", ex);
        } catch (IllegalArgumentException ex) {
            logAndPrintError(newStderr, "Invalid zygote arguments", ex);
        } catch (ZygoteSecurityException ex) {
            logAndPrintError(newStderr,
                    "Zygote security policy prevents request: ", ex);
        }

        try {
            if (pid == 0) {
                // in child
                zygoteServer.closeServerSocket();
                IoUtils.closeQuietly(serverPipeFd);
                serverPipeFd = null;
                handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);

                // should never get here, the child is expected to either
                // throw Zygote.MethodAndArgsCaller or exec().
                return true;
            } else {
                // in parent...pid of < 0 means failure
                IoUtils.closeQuietly(childPipeFd);
                childPipeFd = null;
                return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
            }
        } finally {
            IoUtils.closeQuietly(childPipeFd);
            IoUtils.closeQuietly(serverPipeFd);
        }
    }