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Android框架简介

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|-- MODU......

 

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

 

01 /*
02  * Copyright (C) 2007 The Android Open Source Project
03  *
04  * Licensed under the Apache License, Version 2.0 (the "License");
05  * you may not use this file except in compliance with the License.
06  * You may obtain a copy of the License at
07  *
08  *      http://www.apache.org/licenses/LICENSE-2.0
09  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */ 
16 package com.example.Android.helloactivity; 
17 import Android.app.Activity; 
18 import Android.os.Bundle; 
19 /**
20  * A minimal "Hello, World!" application.
21  */ 
22 public class HelloActivity extends Activity { 
23     public HelloActivity() { 
24     } 
25     /**
26      * Called with the activity is first created.
27      */
28     @Override
29     public void onCreate(Bundle savedInstanceState) { 
30         super.onCreate(savedInstanceState); 
31         // Set the layout for this activity.  You can find it  
32         // in res/layout/hello_activity.xml  
33         setContentView(R.layout.hello_activity); 
34     } 
35 }

 

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

 

1 import Android.app.Activity; 
2 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

01 public class Activity extends ContextThemeWrapper 
02         implements LayoutInflater.Factory, 
03         Window.Callback, KeyEvent.Callback, 
04         OnCreateContextMenuListener, ComponentCallbacks { 
05     private static final String TAG = "Activity"; 
06     /** Standard activity result: operation canceled. */
07     public static final int RESULT_CANCELED    = 0; 
08     /** Standard activity result: operation succeeded. */
09     public static final int RESULT_OK           = -1; 
10     /** Start of user-defined activity results. */
11     public static final int RESULT_FIRST_USER   = 1; 
12     private static long sInstanceCount = 0;

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

1 public final class Bundle implements Parcelable, Cloneable { 
2     private static final String LOG_TAG = "Bundle"; 
3     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,

001 /**
002  * Copyright (C) 2006 The Android Open Source Project
003  *
004  * Licensed under the Apache License, Version 2.0 (the "License");
005  * you may not use this file except in compliance with the License.
006  * You may obtain a copy of the License at
007  *
008  *      http://www.apache.org/licenses/LICENSE-2.0
009  *
010  * Unless required by applicable law or agreed to in writing, software
011  * distributed under the License is distributed on an "AS IS" BASIS,
012  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013  * See the License for the specific language governing permissions and
014  * limitations under the License.
015  */
016 package Android.util; 
017 import com.Android.internal.os.RuntimeInit; 
018 import java.io.PrintWriter; 
019 import java.io.StringWriter; 
020 /**
021  * API for sending log output.
022  *
023  * <p>Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e()
024  * methods.
025  *
026  * <p>The order in terms of verbosity, from least to most is
027  * ERROR, WARN, INFO, DEBUG, VERBOSE.  Verbose should never be compiled
028  * into an application except during development.  Debug logs are compiled
029  * in but stripped at runtime.  Error, warning and info logs are always kept.
030  *
031  * <p><b>Tip:</b> A good convention is to declare a <code>TAG</code> constant
032  * in your class:
033  *
034  * <pre>private static final String TAG = "MyActivity";</pre>
035  *
036  * and use that in subsequent calls to the log methods.
037  * </p>
038  *
039  * <p><b>Tip:</b> Don't forget that when you make a call like
040  * <pre>Log.v(TAG, "index=" + i);</pre>
041  * that when you're building the string to pass into Log.d, the compiler uses a
042  * StringBuilder and at least three allocations occur: the StringBuilder
043  * itself, the buffer, and the String object.  Realistically, there is also
044  * another buffer allocation and copy, and even more pressure on the gc.
045  * That means that if your log message is filtered out, you might be doing
046  * significant work and incurring significant overhead.
047  */
048 public final class Log { 
049     /**
050      * Priority constant for the println method; use Log.v.
051      */
052     public static final int VERBOSE = 2; 
053     /**
054      * Priority constant for the println method; use Log.d.
055      */
056     public static final int DEBUG = 3; 
057     /**
058      * Priority constant for the println method; use Log.i.
059      */
060     public static final int INFO = 4; 
061     /**
062      * Priority constant for the println method; use Log.w.
063      */
064     public static final int WARN = 5; 
065     /**
066      * Priority constant for the println method; use Log.e.
067      */
068     public static final int ERROR = 6; 
069     /**
070      * Priority constant for the println method.
071      */
072     public static final int ASSERT = 7; 
073     /**
074      * Exception class used to capture a stack trace in {@link #wtf()}.
075      */
076     private static class TerribleFailure extends Exception { 
077         TerribleFailure(String msg, Throwable cause) { super(msg, cause); } 
078     } 
079     private Log() { 
080     } 
081     /**
082      * Send a {@link #VERBOSE} log message.
083      * @param tag Used to identify the source of a log message.  It usually identifies
084      *        the class or activity where the log call occurs.
085      * @param msg The message you would like logged.
086      */
087     public static int v(String tag, String msg) { 
088         return println_native(LOG_ID_MAIN, VERBOSE, tag, msg); 
089     } 
090     /**
091      * Send a {@link #VERBOSE} log message and log the exception.
092      * @param tag Used to identify the source of a log message.  It usually identifies
093      *        the class or activity where the log call occurs.
094      * @param msg The message you would like logged.
095      * @param tr An exception to log
096      */
097     public static int v(String tag, String msg, Throwable tr) { 
098         return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '/n' + getStackTraceString(tr)); 
099     } 
100     /**
101      * Send a {@link #DEBUG} log message.
102      * @param tag Used to identify the source of a log message.  It usually identifies
103      *        the class or activity where the log call occurs.
104      * @param msg The message you would like logged.
105      */
106     public static int d(String tag, String msg) { 
107         return println_native(LOG_ID_MAIN, DEBUG, tag, msg); 
108     } 
109     /**
110      * Send a {@link #DEBUG} log message and log the exception.
111      * @param tag Used to identify the source of a log message.  It usually identifies
112      *        the class or activity where the log call occurs.
113      * @param msg The message you would like logged.
114      * @param tr An exception to log
115      */
116     public static int d(String tag, String msg, Throwable tr) { 
117         return println_native(LOG_ID_MAIN, DEBUG, tag, msg + '/n' + getStackTraceString(tr)); 
118     } 
119     /**
120      * Send an {@link #INFO} log message.
121      * @param tag Used to identify the source of a log message.  It usually identifies
122      *        the class or activity where the log call occurs.
123      * @param msg The message you would like logged.
124      */
125     public static int i(String tag, String msg) { 
126         return println_native(LOG_ID_MAIN, INFO, tag, msg); 
127     } 
128     /**
129      * Send a {@link #INFO} log message and log the exception.
130      * @param tag Used to identify the source of a log message.  It usually identifies
131      *        the class or activity where the log call occurs.
132      * @param msg The message you would like logged.
133      * @param tr An exception to log
134      */
135     public static int i(String tag, String msg, Throwable tr) { 
136         return println_native(LOG_ID_MAIN, INFO, tag, msg + '/n' + getStackTraceString(tr)); 
137     } 
138     /**
139      * Send a {@link #WARN} log message.
140      * @param tag Used to identify the source of a log message.  It usually identifies
141      *        the class or activity where the log call occurs.
142      * @param msg The message you would like logged.
143      */
144     public static int w(String tag, String msg) { 
145         return println_native(LOG_ID_MAIN, WARN, tag, msg); 
146     } 
147     /**
148      * Send a {@link #WARN} log message and log the exception.
149      * @param tag Used to identify the source of a log message.  It usually identifies
150      *        the class or activity where the log call occurs.
151      * @param msg The message you would like logged.
152      * @param tr An exception to log
153      */
154     public static int w(String tag, String msg, Throwable tr) { 
155         return println_native(LOG_ID_MAIN, WARN, tag, msg + '/n' + getStackTraceString(tr)); 
156     } 
157     /**
158      * Checks to see whether or not a log for the specified tag is loggable at the specified level.
159      *
160      *  The default level of any tag is set to INFO. This means that any level above and including
161      *  INFO will be logged. Before you make any calls to a logging method you should check to see
162      *  if your tag should be logged. You can change the default level by setting a system property:
163      *      'setprop log.tag.<YOUR_LOG_TAG> <LEVEL>'
164      *  Where level is either VERBOSE, DEBUG, INFO, WARN, ERROR, ASSERT, or SUPPRESS. SUPPRESS will
165      *  turn off all logging for your tag. You can also create a local.prop file that with the
166      *  following in it:
167      *      'log.tag.<YOUR_LOG_TAG>=<LEVEL>'
168      *  and place that in /data/local.prop.
169      *
170      * @param tag The tag to check.
171      * @param level The level to check.
172      * @return Whether or not that this is allowed to be logged.
173      * @throws IllegalArgumentException is thrown if the tag.length() > 23.
174      */
175     public static native boolean isLoggable(String tag, int level); 
176     /**
177      * Send a {@link #WARN} log message and log the exception.
178      * @param tag Used to identify the source of a log message.  It usually identifies
179      *        the class or activity where the log call occurs.
180      * @param tr An exception to log
181      */
182     public static int w(String tag, Throwable tr) { 
183         return println_native(LOG_ID_MAIN, WARN, tag, getStackTraceString(tr)); 
184     } 
185     /**
186      * Send an {@link #ERROR} log message.
187      * @param tag Used to identify the source of a log message.  It usually identifies
188      *        the class or activity where the log call occurs.
189      * @param msg The message you would like logged.
190      */
191     public static int e(String tag, String msg) { 
192         return println_native(LOG_ID_MAIN, ERROR, tag, msg); 
193     } 
194     /**
195      * Send a {@link #ERROR} log message and log the exception.
196      * @param tag Used to identify the source of a log message.  It usually identifies
197      *        the class or activity where the log call occurs.
198      * @param msg The message you would like logged.
199      * @param tr An exception to log
200      */
201     public static int e(String tag, String msg, Throwable tr) { 
202         return println_native(LOG_ID_MAIN, ERROR, tag, msg + '/n' + getStackTraceString(tr)); 
203     } 
204     /**
205      * What a Terrible Failure: Report a condition that should never happen.
206      * The error will always be logged at level ASSERT with the call stack.
207      * Depending on system configuration, a report may be added to the
208      * {@link Android.os.DropBoxManager} and/or the process may be terminated
209      * immediately with an error dialog.
210      * @param tag Used to identify the source of a log message.
211      * @param msg The message you would like logged.
212      */
213     public static int wtf(String tag, String msg) { 
214         return wtf(tag, msg, null); 
215     } 
216     /**
217      * What a Terrible Failure: Report an exception that should never happen.
218      * Similar to {@link #wtf(String, String)}, with an exception to log.
219      * @param tag Used to identify the source of a log message.
220      * @param tr An exception to log.
221      */
222     public static int wtf(String tag, Throwable tr) { 
223         return wtf(tag, tr.getMessage(), tr); 
224     } 
225     /**
226      * What a Terrible Failure: Report an exception that should never happen.
227      * Similar to {@link #wtf(String, Throwable)}, with a message as well.
228      * @param tag Used to identify the source of a log message.
229      * @param msg The message you would like logged.
230      * @param tr An exception to log.  May be null.
231      */
232     public static int wtf(String tag, String msg, Throwable tr) { 
233         tr = new TerribleFailure(msg, tr); 
234         int bytes = println_native(LOG_ID_MAIN, ASSERT, tag, getStackTraceString(tr)); 
235         RuntimeInit.wtf(tag, tr); 
236         return bytes; 
237     } 
238     /**
239      * Handy function to get a loggable stack trace from a Throwable
240      * @param tr An exception to log
241      */
242     public static String getStackTraceString(Throwable tr) { 
243         if (tr == null) { 
244             return ""; 
245         } 
246         StringWriter sw = new StringWriter(); 
247         PrintWriter pw = new PrintWriter(sw); 
248         tr.printStackTrace(pw); 
249         return sw.toString(); 
250     } 
251     /**
252      * Low-level logging call.
253      * @param priority The priority/type of this log message
254      * @param tag Used to identify the source of a log message.  It usually identifies
255      *        the class or activity where the log call occurs.
256      * @param msg The message you would like logged.
257      * @return The number of bytes written.
258      */
259     public static int println(int priority, String tag, String msg) { 
260         return println_native(LOG_ID_MAIN, priority, tag, msg); 
261     } 
262     /** @hide */ public static final int LOG_ID_MAIN = 0; 
263     /** @hide */ public static final int LOG_ID_RADIO = 1; 
264     /** @hide */ public static final int LOG_ID_EVENTS = 2; 
265     /** @hide */ public static final int LOG_ID_SYSTEM = 3; 
266     /** @hide */ public static native int println_native(int bufID, 
267             int priority, String tag, String msg); 
268 }

 

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

 

01 /*
02  * In class Android.util.Log:
03  *  public static native int println_native(int buffer, int priority, String tag, String msg)
04  */
05 static jint Android_util_Log_println_native(JNIEnv* env, jobject clazz, 
06         jint bufID, jint priority, jstring tagObj, jstring msgObj) 
07
08     const char* tag = NULL; 
09     const char* msg = NULL; 
10     if (msgObj == NULL) { 
11         jclass npeClazz; 
12         npeClazz = env->FindClass("java/lang/NullPointerException"); 
13         assert(npeClazz != NULL); 
14         env->ThrowNew(npeClazz, "println needs a message"); 
15         return -1; 
16     } 
17     if (bufID < 0 || bufID >= LOG_ID_MAX) { 
18         jclass npeClazz; 
19         npeClazz = env->FindClass("java/lang/NullPointerException"); 
20         assert(npeClazz != NULL); 
21         env->ThrowNew(npeClazz, "bad bufID"); 
22         return -1; 
23     } 
24     if (tagObj != NULL) 
25         tag = env->GetStringUTFChars(tagObj, NULL); 
26     msg = env->GetStringUTFChars(msgObj, NULL); 
27     int res = __Android_log_buf_write(bufID, (android_LogPriority)priority, tag, msg); 
28     if (tag != NULL) 
29         env->ReleaseStringUTFChars(tagObj, tag); 
30     env->ReleaseStringUTFChars(msgObj, msg); 
31     return res; 
32 }

当然我们发现最终输出是

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

用力grep了一下代码,结果如下

1 ./system/core/include/cutils/log.h:int __Android_log_buf_write(int bufID, intprio, const char *tag, const char *text);
2 ./system/core/liblog/logd_write.c:int __Android_log_buf_write(int bufID, intprio, const char *tag, const char *msg)
3 ./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??