Android 框架简介--Java环境(转)

==========================上===========================

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，

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

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

============================中===========================

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

1./*
2. * Copyright (C) 2007 The Android Open Source Project
3. *
4. * Licensed under the Apache License, Version 2.0 (the "License");
5. * you may not use this file except in compliance with the License.
6. * You may obtain a copy of the License at
7. *
8. *      http://www.apache.org/licenses/LICENSE-2.0
9. *
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

1.public class Activity extends ContextThemeWrapper
2. implements LayoutInflater.Factory,

3.        Window.Callback, KeyEvent.Callback,
4.        OnCreateContextMenuListener, ComponentCallbacks {
5.    private static final String TAG = "Activity"; 6.    /** Standard activity result: operation canceled. */ 7.    public static final int RESULT_CANCELED    = 0; 8.    /** Standard activity result: operation succeeded. */ 9.    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。

=========================下===================

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，

1./*
2. * Copyright (C) 2006 The Android Open Source Project
3. *
4. * Licensed under the Apache License, Version 2.0 (the "License");
5. * you may not use this file except in compliance with the License.
6. * You may obtain a copy of the License at
7. *
8. * http://www.apache.org/licenses/LICENSE-2.0
9. *
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 android.util; 17.import com.android.internal.os.RuntimeInit; 18.import java.io.PrintWriter; 19.import java.io.StringWriter; 20./** 21. * API for sending log output.
22. *
23. * Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e()
24. * methods.
25. *
26. * The order in terms of verbosity, from least to most is
27. * ERROR, WARN, INFO, DEBUG, VERBOSE. Verbose should never be compiled
28. * into an application except during development. Debug logs are compiled
29. * in but stripped at runtime. Error, warning and info logs are always kept.
30. *
31. * Tip: A good convention is to declare a <code>TAG</code> constant
32. * in your class:
33. *
34. * <pre>private static final String TAG = "MyActivity";</pre>
35. *
36. * and use that in subsequent calls to the log methods.
37. * 
38. *
39. * Tip: Don't forget that when you make a call like
40. * <pre>Log.v(TAG, "index=" + i);</pre>
41. * that when you're building the string to pass into Log.d, the compiler uses a
42. * StringBuilder and at least three allocations occur: the StringBuilder
43. * itself, the buffer, and the String object. Realistically, there is also
44. * another buffer allocation and copy, and even more pressure on the gc.
45. * That means that if your log message is filtered out, you might be doing
46. * significant work and incurring significant overhead.
47. */
48.public final class Log {

49. /**

50.     * Priority constant for the println method; use Log.v.
51.     */
52.    public static final int VERBOSE = 2;

53. /**

54.     * Priority constant for the println method; use Log.d.
55.     */
56.    public static final int DEBUG = 3;

57. /**

58.     * Priority constant for the println method; use Log.i.
59.     */
60.    public static final int INFO = 4;

61. /**

62.     * Priority constant for the println method; use Log.w.
63.     */
64.    public static final int WARN = 5;

65. /**

66.     * Priority constant for the println method; use Log.e.
67.     */
68.    public static final int ERROR = 6;

69. /**

70.     * Priority constant for the println method.
71.     */
72.    public static final int ASSERT = 7;

73. /**

74.     * Exception class used to capture a stack trace in {@link #wtf()}.
75.     */
76.    private static class TerribleFailure extends Exception {

77. TerribleFailure(String msg, Throwable cause) { super(msg, cause); } 78. }
79. private Log() {

80. }
81. /**

82.     * Send a {@link #VERBOSE} log message.
83.     * @param tag Used to identify the source of a log message. It usually identifies
84.     *        the class or activity where the log call occurs.
85.     * @param msg The message you would like logged.
86.     */
87.    public static int v(String tag, String msg) {

88. return println_native(LOG_ID_MAIN, VERBOSE, tag, msg);

89. }
90. /**

91.     * Send a {@link #VERBOSE} log message and log the exception.
92.     * @param tag Used to identify the source of a log message. It usually identifies
93.     *        the class or activity where the log call occurs.
94.     * @param msg The message you would like logged.
95.     * @param tr An exception to log
96.     */
97.    public static int v(String tag, String msg, Throwable tr) {

98. return println_native(LOG_ID_MAIN, VERBOSE, tag, msg + '/n' + getStackTraceString(tr));

99. }
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

1./*
2. * In class android.util.Log:
3. * public static native int println_native(int buffer, int priority, String tag, String msg)
4. */
5.static jint android_util_Log_println_native(JNIEnv* env, jobject clazz,

6. jint bufID, jint priority, jstring tagObj, jstring msgObj)
7.{
8. const char* tag = NULL;

9. 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

./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？？

本篇文章来源于 Linux公社网站(www.linuxidc.com) 原文链接：http://www.linuxidc.com/Linux/2011-10/44959p3.htm

posted @ 2013-09-03 20:05 Salesforce 阅读(404) 评论(0) 编辑收藏举报

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