dvm，app进程，linux进程三者关系
DVM指 dalivk 的虚拟机。每一个 Android 应用程序都在它自己的进程中运行，都拥有一个独立的 Dalvik 虚拟机实例。而每一个 DVM 都是在 Linux 中的一个进程，所以说可以认为是同一个概念
Zygote进程与app进程关系
Zygote是java层的进程即它也拥有一个独立的Dalvik 虚拟机实例，它是被linux层的第一个用户空间Init进程所启动的，它的主要作用就是用来孵化app进程和系统进程
fork一个app进程，是通过ActivityManagerService类向Zygote发出fork命令，ActivityManagerService是在系统进程，但是Zygote处于自己的进程中，它们之间的通信没有采用binder机制，而是采用了socket机制，因此我们可以把Zygote称为一个孵化server，ActivityMamagerService称为一个client
下面的图描述了上面的过程
fork进程
涉及到的类
我们先来梳理这个过程中使用到的类，并且这些类是做什么的
涉及到的类
以server和client2个维度来归纳这些类
Zygote进程启动分析
Zygote进程启动后，ZygoteInit类的main方法会被执行
public static void main(String argv[]) {
try {
// Start profiling the zygote initialization.
SamplingProfilerIntegration.start();
boolean startSystemServer = false;
String socketName = "zygote";
String abiList = null;
for (int i = 1; i < argv.length; i++) {
if ("start-system-server".equals(argv[i])) {
startSystemServer = true;
} else if (argv[i].startsWith(ABI_LIST_ARG)) {
abiList = argv[i].substring(ABI_LIST_ARG.length());
} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
socketName = argv[i].substring(SOCKET_NAME_ARG.length());
} else {
throw new RuntimeException("Unknown command line argument: " + argv[i]);
}
}
if (abiList == null) {
throw new RuntimeException("No ABI list supplied.");
}
/*启动servier socket*/
registerZygoteSocket(socketName);
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
SystemClock.uptimeMillis());
preload();
EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END,
SystemClock.uptimeMillis());
// Finish profiling the zygote initialization.
SamplingProfilerIntegration.writeZygoteSnapshot();
// Do an initial gc to clean up after startup
gc();
// Disable tracing so that forked processes do not inherit stale tracing tags from
// Zygote.
Trace.setTracingEnabled(false);
if (startSystemServer) {
/*启动系统服务*/
startSystemServer(abiList, socketName);
}
Log.i(TAG, "Accepting command socket connections");
runSelectLoop(abiList);
closeServerSocket();
} catch (MethodAndArgsCaller caller) {
//这行代码很重要
caller.run();
} catch (RuntimeException ex) {
Log.e(TAG, "Zygote died with exception", ex);
closeServerSocket();
throw ex;
}
}
上面代码主要做了下面的事情:
-
registerZygoteSocket(socketName)启动一个ServerSocket -
preload()预加载资源，预加载耗时的类 -
startSystemServer(abiList, socketName)启动系统服务，并且fork系统进程 -
runSelectLoop(abiList)监听client socket的连接
来看下registerZygoteSocket(socketName)方法
private static void registerZygoteSocket(String socketName) {
if (sServerSocket == null) {
int fileDesc;
final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;
try {
String env = System.getenv(fullSocketName);
fileDesc = Integer.parseInt(env);
} catch (RuntimeException ex) {
throw new RuntimeException(fullSocketName + " unset or invalid", ex);
}
try {
sServerSocket = new LocalServerSocket(
createFileDescriptor(fileDesc));
} catch (IOException ex) {
throw new RuntimeException(
"Error binding to local socket '" + fileDesc + "'", ex);
}
}
}
代码很简单，再来看下preload()方法
static void preload() {
Log.d(TAG, "begin preload");
preloadClasses();
preloadResources();
preloadOpenGL();
preloadSharedLibraries();
// Ask the WebViewFactory to do any initialization that must run in the zygote process,
// for memory sharing purposes.
WebViewFactory.prepareWebViewInZygote();
Log.d(TAG, "end preload");
}
预加载耗时的类，预加载资源等等其他的预加载工作,我们简单看下preloadClasses()和preloadResources()所做的事情
private static void preloadClasses() {
.......省略代码
is = new FileInputStream(PRELOADED_CLASSES);
......省略代码
BufferedReader br
= new BufferedReader(new InputStreamReader(is), 256);
int count = 0;
String line;
while ((line = br.readLine()) != null) {
// Skip comments and blank lines.
line = line.trim();
if (line.startsWith("#") || line.equals("")) {
continue;
}
try {
if (false) {
Log.v(TAG, "Preloading " + line + "...");
}
Class.forName(line);
if (Debug.getGlobalAllocSize() > PRELOAD_GC_THRESHOLD) {
if (false) {
Log.v(TAG,
" GC at " + Debug.getGlobalAllocSize());
}
System.gc();
runtime.runFinalizationSync();
Debug.resetGlobalAllocSize();
}
count++;
......省略代码
}
......省略代码
}
private static void preloadResources() {
final VMRuntime runtime = VMRuntime.getRuntime();
Debug.startAllocCounting();
try {
System.gc();
runtime.runFinalizationSync();
mResources = Resources.getSystem();
mResources.startPreloading();
if (PRELOAD_RESOURCES) {
Log.i(TAG, "Preloading resources...");
long startTime = SystemClock.uptimeMillis();
TypedArray ar = mResources.obtainTypedArray(
com.android.internal.R.array.preloaded_drawables);
int N = preloadDrawables(runtime, ar);
ar.recycle();
Log.i(TAG, "...preloaded " + N + " resources in "
+ (SystemClock.uptimeMillis()-startTime) + "ms.");
startTime = SystemClock.uptimeMillis();
ar = mResources.obtainTypedArray(
com.android.internal.R.array.preloaded_color_state_lists);
N = preloadColorStateLists(runtime, ar);
ar.recycle();
Log.i(TAG, "...preloaded " + N + " resources in "
+ (SystemClock.uptimeMillis()-startTime) + "ms.");
}
mResources.finishPreloading();
} catch (RuntimeException e) {
Log.w(TAG, "Failure preloading resources", e);
} finally {
Debug.stopAllocCounting();
}
}
preloadClasses方法所做的事情是从"/system/etc/preloaded-classes"文件种把预加载的类加载到虚拟机中
在来看runSelectLoop方法
private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();
ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();
FileDescriptor[] fdArray = new FileDescriptor[4];
fds.add(sServerSocket.getFileDescriptor());
peers.add(null);
int loopCount = GC_LOOP_COUNT;
while (true) {
int index;
/*
* Call gc() before we block in select().
* It's work that has to be done anyway, and it's better
* to avoid making every child do it. It will also
* madvise() any free memory as a side-effect.
*
* Don't call it every time, because walking the entire
* heap is a lot of overhead to free a few hundred bytes.
*/
if (loopCount <= 0) {
gc();
loopCount = GC_LOOP_COUNT;
} else {
loopCount--;
}
try {
fdArray = fds.toArray(fdArray);
index = selectReadable(fdArray);
} catch (IOException ex) {
throw new RuntimeException("Error in select()", ex);
}
if (index < 0) {
throw new RuntimeException("Error in select()");
} else if (index == 0) {
ZygoteConnection newPeer = acceptCommandPeer(abiList);
peers.add(newPeer);
fds.add(newPeer.getFileDescriptor());
} else {
boolean done;
/*开始读取client发出的命令*/
done = peers.get(index).runOnce();
if (done) {
peers.remove(index);
fds.remove(index);
}
}
}
}
它所做的事情是：
- 监听client的socket连接
- 发现有连接则建立一个ZygoteConnection对象
- client发送命令，则找到相应的ZygoteConnection对象，并且调用该对象的runOnce方法，来处理client发送的命令
- ZygoteConnection对象处理完毕，则从列表中移除
关于Zygote进程内容介绍到这，后面介绍app进程的fork过程
