Android 框架简介

这篇文章写的挺好的，适合有一定编程基础的人学习Android，遂转之！

 

======================= 第一节 ===========================

这里简单的介绍了Android的java环境基础，在后面一节中会结合具体的实例来理解这一节的内容。

一、Dalvik虚拟机

Dalvik是Android的程序的java虚拟机，代码在dalvik/下，

./
|-- Android.mk 
|-- CleanSpec.mk
|-- MODULE_LICENSE_APACHE2
|-- NOTICE

|-- README.txt 
|-- dalvikvm 虚拟机的实现库   
|-- dexdump   
|-- dexlist 
|-- dexopt 
|-- docs 
|-- dvz 
|-- dx 
|-- hit 
|-- libcore 
|-- libcore-disabled 
|-- libdex 
|-- libnativehelper 使用JNI调用本地代码时用到这个库 
|-- run-core-tests.sh 
|-- tests 
|-- tools 
`-- vm

二、Android的java框架

Android层次中第3层是java框架，第四层就是java应用程序。

Android的java类代码，主要是在frameworks/base/core/java/下，

./
|-- Android
|-- com
|-- jarjar-rules.txt
`-- overview.html

我们再看一下frameworks/base/目录

./
|-- Android.mk
|-- CleanSpec.mk
|-- MODULE_LICENSE_APACHE2
|-- NOTICE
|-- api
|-- awt
|-- build
|-- camera
|-- cmds
|-- common
|-- core
|-- data
|-- docs
|-- graphics
|-- include
|-- keystore
|-- libs
|-- location
|-- media
|-- native
|-- obex
|-- opengl
|-- packages
|-- preloaded-classes
|-- sax
|-- services
|-- telephony
|-- test-runner
|-- tests
|-- tools
|-- vpn
`-- wifi

这里也有Android的java框架代码。

三、JNI

在Android中，通过JNI，java可以调用C写的代码，主要的实现是在frameworks/base/core/jni，通过查看Android.mk，我们可以看到最后生成了libandroid_runtime.so，具体实现JNI功能需要上面我们介绍的libnativehelper.so，

四、系统服务之java

1、binder，提供Android的IPC功能

2、servicemanager，服务管理的服务器端

3、系统进程zygote，负责孵化所有的新应用

======================= 第二节 ==========================

在我平时工作中主要是进行linux网络子系统的模块开发、linux应用程序（C/C++）开发。在学习和从事驱动模块开发的过程中，如果你对linux系统本身，包括应用程序开发都不了解，那么读内核代码就如同天书，毫无意义，所以我分析框架也是从基本系统api开始的，当然也不会太多涉及到应用程序开发。

好，开始这节主要是讲一个简单的adnroid应用程序，从应用程序出发，到框架代码。

分析的应用程序我们也奉行拿来主义:froyo/development/samples/HelloActivity

./
|-- Android.mk
|-- AndroidManifest.xml
|-- res
|-- src
`-- tests

其他的就多说了，看代码

/*  * Copyright (C) 2007 The Android Open Source Project  *  * Licensed under the Apache License, Version 2.0 (the "License");  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  *  *      http://www.apache.org/licenses/LICENSE-2.0  *  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an "AS IS" BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */  package com.example.Android.helloactivity;  import Android.app.Activity;  import Android.os.Bundle;  /**  * A minimal "Hello, World!" application.  */  public class HelloActivity extends Activity {      public HelloActivity() {      }      /**      * Called with the activity is first created.      */      @Override      public void onCreate(Bundle savedInstanceState) {          super.onCreate(savedInstanceState);          // Set the layout for this activity.  You can find it           // in res/layout/hello_activity.xml           setContentView(R.layout.hello_activity);      }  }   

每一个写过Android程序的人都应该是从这个代码起步的吧？那好，那么我们研究android框架也从这里启航。

首先是

import Android.app.Activity;  import Android.os.Bundle;  

记住，我们这里不是讲JAVA，我们要讲的是Android.app.Activity，回顾上节的内容，android的JAVA框架代码放在froyo/frameworks/base/，

其中Activity的代码放在框架代码的core/java/Android/app/Activity.java，大概看一下

public class Activity extends ContextThemeWrapper          implements LayoutInflater.Factory,          Window.Callback, KeyEvent.Callback,          OnCreateContextMenuListener, ComponentCallbacks {      private static final String TAG = "Activity";      /** Standard activity result: operation canceled. */      public static final int RESULT_CANCELED    = 0;      /** Standard activity result: operation succeeded. */      public static final int RESULT_OK           = -1;      /** Start of user-defined activity results. */      public static final int RESULT_FIRST_USER   = 1;      private static long sInstanceCount = 0;  

同样的Bundle的代码core/java/Android/os/Bundle.java

public final class Bundle implements Parcelable, Cloneable {      private static final String LOG_TAG = "Bundle";      public static final Bundle EMPTY;  

呵呵，其实写多应用程序，然后看看这些代码，会有更加豁然开朗的感觉，所以列出以上目录给大家参考，所有的java框架代码都在那个目录下，到这里今天要讨论的第一个问题就到这里了。

我所在的公司是网络设备供应商，其实和Android本身不搭边，android只是平时的爱好而已，所以很多地方如果写错了敬请原谅，当然也计划去做做android系统开发，例如驱动或者是框架开发，这是后话。

======================== 第三节 ========================

上节讲到了JAVA框架代码和应用程序的关系，那么框架代码和驱动层是怎么联系的呢？这就是这一节的内容：JNI

java使用一种叫做jni的技术来支持对C/C++代码的调用，在anroid中jni的代码放在froyo/frameworks/base/core/jni下，当然在java框架代码的目录下还有其他地方也多多少少放了jni代码，大家可以打开源码来看看。

整体关系如下图：

| java应用程序

--------------------------------------- Android系统api

| java框架

    ｜本地接口声明

--------------------------------------

| JNI
--------------------------------------

| C/C++代码

继续拿来主义，C/C++中调试用printf，内核调试用printk，呵呵，Android调试用log，那么我们就分析log的实现。

log的java代码froyo/frameworks/base/core/java/Android/util/Log.java，

/**  * Copyright (C) 2006 The Android Open Source Project  *  * Licensed under the Apache License, Version 2.0 (the "License");  * you may not use this file except in compliance with the License.  * You may obtain a copy of the License at  *  *      http://www.apache.org/licenses/LICENSE-2.0  *  * Unless required by applicable law or agreed to in writing, software  * distributed under the License is distributed on an "AS IS" BASIS,  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  * See the License for the specific language governing permissions and  * limitations under the License.  */  package Android.util;  import com.Android.internal.os.RuntimeInit;  import java.io.PrintWriter;  import java.io.StringWriter;  /**  * API for sending log output.  *  * <p>Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e()  * methods.  *  * <p>The order in terms of verbosity, from least to most is  * ERROR, WARN, INFO, DEBUG, VERBOSE.  Verbose should never be compiled  * into an application except during development.  Debug logs are compiled  * in but stripped at runtime.  Error, warning and info logs are always kept.  *  * <p><b>Tip:</b> A good convention is to declare a <code>TAG</code> constant  * in your class:  *  * <pre>private static final String TAG = "MyActivity";</pre>  *  * and use that in subsequent calls to the log methods.  * </p>  *  * <p><b>Tip:</b> Don't forget that when you make a call like  * <pre>Log.v(TAG, "index=" + i);</pre>  * that when you're building the string to pass into Log.d, the compiler uses a  * StringBuilder and at least three allocations occur: the StringBuilder  * itself, the buffer, and the String object.  Realistically, there is also  * another buffer allocation and copy, and even more pressure on the gc.  * That means that if your log message is filtered out, you might be doing  * significant work and incurring significant overhead.  */  public final class Log {      /**      * Priority constant for the println method; use Log.v.      */      public static final int VERBOSE = 2;      /**      * Priority constant for the println method; use Log.d.      */      public static final int DEBUG = 3;      /**      * Priority constant for the println method; use Log.i.      */      public static final int INFO = 4;      /**      * Priority constant for the println method; use Log.w.      */      public static final int WARN = 5;      /**      * Priority constant for the println method; use Log.e.      */      public static final int ERROR = 6;      /**      * Priority constant for the println method.      */      public static final int ASSERT = 7;      /**      * Exception class used to capture a stack trace in {@link #wtf()}.      */      private static class TerribleFailure extends Exception {          TerribleFailure(String msg, Throwable cause) { super(msg, cause); }      }      private Log() {      }      /**      * Send a {@link #VERBOSE} log message.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      */      public static int v(String tag, String msg) {          return println_native(LOG_ID_MAIN, VERBOSE, tag, msg);      }      /**      * Send a {@link #VERBOSE} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @param tr An exception to log      */      public static int v(String tag, String msg, Throwable tr) {          return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '/n' + getStackTraceString(tr));      }      /**      * Send a {@link #DEBUG} log message.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      */      public static int d(String tag, String msg) {          return println_native(LOG_ID_MAIN, DEBUG, tag, msg);      }      /**      * Send a {@link #DEBUG} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @param tr An exception to log      */      public static int d(String tag, String msg, Throwable tr) {          return println_native(LOG_ID_MAIN, DEBUG, tag, msg + '/n' + getStackTraceString(tr));      }      /**      * Send an {@link #INFO} log message.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      */      public static int i(String tag, String msg) {          return println_native(LOG_ID_MAIN, INFO, tag, msg);      }      /**      * Send a {@link #INFO} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @param tr An exception to log      */      public static int i(String tag, String msg, Throwable tr) {          return println_native(LOG_ID_MAIN, INFO, tag, msg + '/n' + getStackTraceString(tr));      }      /**      * Send a {@link #WARN} log message.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      */      public static int w(String tag, String msg) {          return println_native(LOG_ID_MAIN, WARN, tag, msg);      }      /**      * Send a {@link #WARN} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @param tr An exception to log      */      public static int w(String tag, String msg, Throwable tr) {          return println_native(LOG_ID_MAIN, WARN, tag, msg + '/n' + getStackTraceString(tr));      }      /**      * Checks to see whether or not a log for the specified tag is loggable at the specified level.      *      *  The default level of any tag is set to INFO. This means that any level above and including      *  INFO will be logged. Before you make any calls to a logging method you should check to see      *  if your tag should be logged. You can change the default level by setting a system property:      *      'setprop log.tag.<YOUR_LOG_TAG> <LEVEL>'      *  Where level is either VERBOSE, DEBUG, INFO, WARN, ERROR, ASSERT, or SUPPRESS. SUPPRESS will      *  turn off all logging for your tag. You can also create a local.prop file that with the      *  following in it:      *      'log.tag.<YOUR_LOG_TAG>=<LEVEL>'      *  and place that in /data/local.prop.      *      * @param tag The tag to check.      * @param level The level to check.      * @return Whether or not that this is allowed to be logged.      * @throws IllegalArgumentException is thrown if the tag.length() > 23.      */      public static native boolean isLoggable(String tag, int level);      /**      * Send a {@link #WARN} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param tr An exception to log      */      public static int w(String tag, Throwable tr) {          return println_native(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr));      }      /**      * Send an {@link #ERROR} log message.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      */      public static int e(String tag, String msg) {          return println_native(LOG_ID_MAIN, ERROR, tag, msg);      }      /**      * Send a {@link #ERROR} log message and log the exception.      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @param tr An exception to log      */      public static int e(String tag, String msg, Throwable tr) {          return println_native(LOG_ID_MAIN, ERROR, tag, msg + '/n' + getStackTraceString(tr));      }      /**      * What a Terrible Failure: Report a condition that should never happen.      * The error will always be logged at level ASSERT with the call stack.      * Depending on system configuration, a report may be added to the      * {@link Android.os.DropBoxManager} and/or the process may be terminated      * immediately with an error dialog.      * @param tag Used to identify the source of a log message.      * @param msg The message you would like logged.      */      public static int wtf(String tag, String msg) {          return wtf(tag, msg, null);      }      /**      * What a Terrible Failure: Report an exception that should never happen.      * Similar to {@link #wtf(String, String)}, with an exception to log.      * @param tag Used to identify the source of a log message.      * @param tr An exception to log.      */      public static int wtf(String tag, Throwable tr) {          return wtf(tag, tr.getMessage(), tr);      }      /**      * What a Terrible Failure: Report an exception that should never happen.      * Similar to {@link #wtf(String, Throwable)}, with a message as well.      * @param tag Used to identify the source of a log message.      * @param msg The message you would like logged.      * @param tr An exception to log.  May be null.      */      public static int wtf(String tag, String msg, Throwable tr) {          tr = new TerribleFailure(msg, tr);          int bytes = println_native(LOG_ID_MAIN, ASSERT, tag, getStackTraceString(tr));          RuntimeInit.wtf(tag, tr);          return bytes;      }      /**      * Handy function to get a loggable stack trace from a Throwable      * @param tr An exception to log      */      public static String getStackTraceString(Throwable tr) {          if (tr == null) {              return "";          }          StringWriter sw = new StringWriter();          PrintWriter pw = new PrintWriter(sw);          tr.printStackTrace(pw);          return sw.toString();      }      /**      * Low-level logging call.      * @param priority The priority/type of this log message      * @param tag Used to identify the source of a log message.  It usually identifies      *        the class or activity where the log call occurs.      * @param msg The message you would like logged.      * @return The number of bytes written.      */      public static int println(int priority, String tag, String msg) {          return println_native(LOG_ID_MAIN, priority, tag, msg);      }      /** @hide */ public static final int LOG_ID_MAIN = 0;      /** @hide */ public static final int LOG_ID_RADIO = 1;      /** @hide */ public static final int LOG_ID_EVENTS = 2;      /** @hide */ public static final int LOG_ID_SYSTEM = 3;      /** @hide */ public static native int println_native(int bufID,              int priority, String tag, String msg);  }  

我们看到所有代码都是调用public static native int println_native(int bufID,
            int priority, String tag, String msg);来实现输出的，这个函数的实现就是C++，调用的方式就是JNI

我们看一下对应的jni代码froyo/frameworks/base/core/jni/Android_util_Log.cpp，最终调用的输出函数是

/*  * In class Android.util.Log:  *  public static native int println_native(int buffer, int priority, String tag, String msg)  */  static jint Android_util_Log_println_native(JNIEnv* env, jobject clazz,          jint bufID, jint priority, jstring tagObj, jstring msgObj)  {      const char* tag = NULL;      const char* msg = NULL;      if (msgObj == NULL) {          jclass npeClazz;          npeClazz = env->FindClass("java/lang/NullPointerException");          assert(npeClazz != NULL);          env->ThrowNew(npeClazz, "println needs a message");          return -1;      }      if (bufID < 0 || bufID >= LOG_ID_MAX) {          jclass npeClazz;          npeClazz = env->FindClass("java/lang/NullPointerException");          assert(npeClazz != NULL);          env->ThrowNew(npeClazz, "bad bufID");          return -1;      }      if (tagObj != NULL)          tag = env->GetStringUTFChars(tagObj, NULL);      msg = env->GetStringUTFChars(msgObj, NULL);      int res = __Android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg);      if (tag != NULL)          env->ReleaseStringUTFChars(tagObj, tag);      env->ReleaseStringUTFChars(msgObj, msg);      return res;  }  

当然我们发现最终输出是

int res = __Android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg);  

用力grep了一下代码，结果如下

./system/core/include/cutils/log.h:int __Android_log_buf_write(int bufID, int prio, const char *tag, const char *text);./system/core/liblog/logd_write.c:int __Android_log_buf_write(int bufID, int prio, const char *tag, const char *msg)./system/core/liblog/logd_write.c:    return __Android_log_buf_write(bufID, prio, tag, buf);

这个就是和Android专用驱动进行通信的方式，这个分析下去就有点深了，后面分析。

以上三个小节分析了Android的JAVA环境，我这里都是简单的抛砖引玉，希望能给大家一点大体的指引，其他修行靠大家了，能成为是一个android程序员是多么幸福的事情，各位已经在幸福中了，我什么时候也可以幸福一把？？