Android 框架简介

D_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

? 1int 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。