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??