Android 框架簡介

這篇文章寫的挺好的，適合有一定編程基礎的人學習Android，遂轉之！

 

======================= 第一節 ===========================

這裏簡單的介紹了Android的java環境基礎，在後面一節中會結合具體的實例來理解這一節的內容。

一、Dalvik虛擬機

Dalvik是Android的程序的java虛擬機，代碼在dalvik/下，

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

|-- README.txt 
|-- dalvikvm 虛擬機的實現庫   
|-- dexdump   
|-- dexlist 
|-- dexopt 
|-- docs 
|-- dvz 
|-- dx 
|-- hit 
|-- libcore 
|-- libcore-disabled 
|-- libdex 
|-- libnativehelper 使用JNI調用本地代碼時用到這個庫 
|-- run-core-tests.sh 
|-- tests 
|-- tools 
`-- vm

二、Android的java框架

Android層次中第3層是java框架，第四層就是java應用程序。

Android的java類代碼，主要是在frameworks/base/core/java/下，

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

我們再看一下frameworks/base/目錄

./
|-- 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

這裏也有Android的java框架代碼。

三、JNI

在Android中，通過JNI，java可以調用C寫的代碼，主要的實現是在frameworks/base/core/jni，通過查看Android.mk，我們可以看到最後生成了libandroid_runtime.so，具體實現JNI功能需要上面我們介紹的libnativehelper.so，

四、系統服務之java

1、binder，提供Android的IPC功能

2、servicemanager，服務管理的服務器端

3、系統進程zygote，負責孵化所有的新應用

======================= 第二節 ==========================

在我平時工作中主要是進行linux網絡子系統的模塊開發、linux應用程序（C/C++）開發。在學習和從事驅動模塊開發的過程中，如果你對linux系統本身，包括應用程序開發都不瞭解，那麼讀內核代碼就如同天書，毫無意義，所以我分析框架也是從基本系統api開始的，當然也不會太多涉及到應用程序開發。

好，開始這節主要是講一個簡單的adnroid應用程序，從應用程序出發，到框架代碼。

分析的應用程序我們也奉行拿來主義:froyo/development/samples/HelloActivity

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

其他的就多說了，看代碼

/*
 * 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);
    }
}

每一個寫過Android程序的人都應該是從這個代碼起步的吧？那好，那麼我們研究android框架也從這裏啓航。

首先是

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

記住，我們這裏不是講JAVA，我們要講的是Android.app.Activity，回顧上節的內容，android的JAVA框架代碼放在froyo/frameworks/base/，

其中Activity的代碼放在框架代碼的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;

同樣的Bundle的代碼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，其實寫多應用程序，然後看看這些代碼，會有更加豁然開朗的感覺，所以列出以上目錄給大家參考，所有的java框架代碼都在那個目錄下，到這裏今天要討論的第一個問題就到這裏了。

我所在的公司是網絡設備供應商，其實和Android本身不搭邊，android只是平時的愛好而已，所以很多地方如果寫錯了敬請原諒，當然也計劃去做做android系統開發，例如驅動或者是框架開發，這是後話。

======================== 第三節 ========================

上節講到了JAVA框架代碼和應用程序的關係，那麼框架代碼和驅動層是怎麼聯繫的呢？這就是這一節的內容：JNI

java使用一種叫做jni的技術來支持對C/C++代碼的調用，在anroid中jni的代碼放在froyo/frameworks/base/core/jni下，當然在java框架代碼的目錄下還有其他地方也多多少少放了jni代碼，大家可以打開源碼來看看。

整體關係如下圖：

| java應用程序

--------------------------------------- Android系統api

| java框架

    ｜本地接口聲明

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

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

| C/C++代碼

繼續拿來主義，C/C++中調試用printf，內核調試用printk，呵呵，Android調試用log，那麼我們就分析log的實現。

log的java代碼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);
}

我們看到所有代碼都是調用public static native int println_native(int bufID,
            int priority, String tag, String msg);來實現輸出的，這個函數的實現就是C++，調用的方式就是JNI

我們看一下對應的jni代碼froyo/frameworks/base/core/jni/Android_util_Log.cpp，最終調用的輸出函數是

/*
 * 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;
}

當然我們發現最終輸出是

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

用力grep了一下代碼，結果如下

./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);

這個就是和Android專用驅動進行通信的方式，這個分析下去就有點深了，後面分析。

以上三個小節分析了Android的JAVA環境，我這裏都是簡單的拋磚引玉，希望能給大家一點大體的指引，其他修行靠大家了，能成爲是一個android程序員是多麼幸福的事情，各位已經在幸福中了，我什麼時候也可以幸福一把？？