Android 框架简介

虫虫 发布于 2011/10/10 11:12
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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??

 

转载自:http://www.linuxidc.com/Linux/2011-10/44959p3.htm

加载中
0
JavaGG
JavaGG
好强大!!
0
J
JAVA_wangzhipeng
对于新手,不太容易懂
0
不想敲代码的码农
不想敲代码的码农

我什么时候能达到楼主这种水平就好了。顶一个!!!

0
x
xhchen

谢谢楼主,什么时候动手实践一下

0
syslight
syslight
哇,好啊,穿针引线之功啊~~~
0
Victor_Amy
Victor_Amy
不大能看懂!
0
TimApache
TimApache
虽然看不太懂,但好强大哦!
0
Michael_Yuan
Michael_Yuan
完全看不懂,天书,请问楼主,我只有C+Linux的基础,怎么入手安卓?
0
tonypacer
tonypacer
我需要再买一部电脑,装LINUX 吗
祝俊红
装个虚拟机不就好咯,何必再买台电脑
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