【Android学习】《Android进阶解密》二之二

从Android 5.0开始，Android开始支持64位程序，Zygote也就有了32位和64位的区别，所以在这里用ro.zygote属性来控制使用不同的Zygote启动脚本，从而也就启动了不同版本的Zygote进程

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
    class main
    priority -20
    user root
    group root readproc
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver
    onrestart restart cameraserver
    onrestart restart media
    onrestart restart netd
    onrestart restart wificond
    writepid /dev/cpuset/foreground/tasks

service zygote /system/bin/app_process32 -Xzygote /system/bin --zygote --start-system-server --socket-name=zygote
    class main
    priority -20
    user root
    group root readproc
    socket zygote stream 660 root system
    onrestart write /sys/android_power/request_state wake
    onrestart write /sys/power/state on
    onrestart restart audioserver
    onrestart restart cameraserver
    onrestart restart media
    onrestart restart netd
    onrestart restart wificond
    writepid /dev/cpuset/foreground/tasks

service zygote_secondary /system/bin/app_process64 -Xzygote /system/bin --zygote --socket-name=zygote_secondary
    class main
    priority -20
    user root
    group root readproc
    socket zygote_secondary stream 660 root system
    onrestart restart zygote
    writepid /dev/cpuset/foreground/tasks

在注释1处调用startVm函数来创建Java虚拟机，在注释2处调用startReg函数为Java虚拟机注册JNI方法。注释3处的className的值是传进来的参数，它的值为com.android.internal.os.ZygoteInit。在注释4处通过toSlashClassName函数，将className的“.”替换为“/”，替换后的值为com/android/internal/os/ZygoteInit，并赋值给slashClassName，接着在注释5处根据slashClassName找到ZygoteInit，找到了ZygoteInit后顺理成章地在注释6处找到ZygoteInit的main方法。最终会在注释7处通过JNI调用ZygoteInit的main方法。这里为何要使用JNI呢？因为ZygoteInit的main方法是由Java语言编写的，当前的运行逻辑在Native中，这就需要通过JNI来调用Java。这样Zygote就从Native层进入了Java框架层。

private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer)
            throws Zygote.MethodAndArgsCaller, RuntimeException { 这玩意创建启动systemserver的启动参数
        long capabilities = posixCapabilitiesAsBits(
            OsConstants.CAP_IPC_LOCK,
            OsConstants.CAP_KILL,
            OsConstants.CAP_NET_ADMIN,
            OsConstants.CAP_NET_BIND_SERVICE,
            OsConstants.CAP_NET_BROADCAST,
            OsConstants.CAP_NET_RAW,
            OsConstants.CAP_SYS_MODULE,
            OsConstants.CAP_SYS_NICE,
            OsConstants.CAP_SYS_PTRACE,
            OsConstants.CAP_SYS_TIME,
            OsConstants.CAP_SYS_TTY_CONFIG,
            OsConstants.CAP_WAKE_ALARM
        );
        /* Containers run without this capability, so avoid setting it in that case */
        if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) {
            capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND);
        }
        /* Hardcoded command line to start the system server */
        String args[] = {
            "--setuid=1000",
            "--setgid=1000",
            "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,1032,3001,3002,3003,3006,3007,3009,3010",
            "--capabilities=" + capabilities + "," + capabilities,
            "--nice-name=system_server",
            "--runtime-args",
            "com.android.server.SystemServer",
        };
        ZygoteConnection.Arguments parsedArgs = null;

        int pid;

        try {
            parsedArgs = new ZygoteConnection.Arguments(args);
            ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
            ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);

            /* Request to fork the system server process */
            pid = Zygote.forkSystemServer( 创建一个子进程 就是systemserver的进程
                    parsedArgs.uid, parsedArgs.gid,
                    parsedArgs.gids,
                    parsedArgs.debugFlags,
                    null,
                    parsedArgs.permittedCapabilities,
                    parsedArgs.effectiveCapabilities);
        } catch (IllegalArgumentException ex) {
            throw new RuntimeException(ex);
        }

        /* For child process */
        if (pid == 0) {
            if (hasSecondZygote(abiList)) {
                waitForSecondaryZygote(socketName);
            }

            zygoteServer.closeServerSocket();
            handleSystemServerProcess(parsedArgs);处理systemserver进程
            
        }

        return true;
    }