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
/**
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* Copyright (C) 2006 The Android Open Source Project
003
*
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* 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
*
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* <p>Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e()
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* methods.
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*
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* <p>The order in terms of verbosity, from least to most is
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* ERROR, WARN, INFO, DEBUG, VERBOSE.Verbose should never be compiled
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* into an application except during development.Debug logs are compiled
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* in but stripped at runtime.Error, warning and info logs are always kept.
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*
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* <p><b>Tip:</b> A good convention is to declare a <code>TAG</code> constant
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* in your class:
033
*
034
* <pre>private static final String TAG = "MyActivity";</pre>
035
*
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* 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??