Android Zygote Startup

Here are some miscellaneous notes on Android startup of the zygote process.

This information is true for eclair, as of August 2010.

Sequence of startup steps

• init runs the C++ program /system/bin/app_process, and gives the resulting process the name "zygote"
  • This is based on the line from init.rc:

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server

• app_process executes, and executes a runtime environment for a dalvik class
  • app_process usage claims to be:

Usage: app_process [java-options] cmd-dir start-class-name [options]

• • but you can specify --zygote in place of the start-class-name.
  • In the bootup case, --zygote is used
  • source for 'app_process is in frameworks/base/cmds/app_process/app_main.cpp
  • this is the program to debug when debugging zygote/dalvik itself
• app_process does a 'runtime.start("com.android.internal.os.ZygoteInit", startSystemServer)
  • the startSystemServer flag is passed as a parameter on to app_process, to indicate whether to start the system server (duh)
• runtime is an instance of class AppRuntime, which is sub-classed from AndroidRuntime
• AndroidRuntime is the main class for starting the dalvik runtime environment
  • source is in frameworks/base/core/jni/AndroidRuntime.cpp
  • the start() method starts the virtual machine
    • LOG_BOOT_PROGRESS_START is emitted, with systemTime(SYSTEM_TIME_MONOTONIC)
    • startVM() is called
    • eventually, callStaticVoidMethod() is called, to actually start executing the start class with method "main" in Dalvik code
• com.android.internal.os.ZygoteInit:main() starts executing
  • source is at: frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
• the profiler is started
• the Zygote socket is registered (for later communication to start apps)
• classes and resources are preloaded
• zygote runs in "select loop mode", where a single process spins waiting for communication to start subsequent apps
  • see runSelectLoopMode()
    • new connections are accepted (and put into array "peers")
    • the spawn command received over the network is executed by calling the runOnce() method
    • source code for this is at: frameworks/base/core/java/com/android/internal/os/ZygoteConnection.java
• eventually, a call is made to Zygote.forkAndSpecialize(), which does the actual forking