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LOG类： public final class Log extends Object java.lang.Object   android.util.Log Constants int ASSERT Priority constant for the println method. int DEBUG Priority constant for the println method; use Log.d. 输出DEBUG故障日志信息 int ERROR Priority constant for the println method; use Log.e. 输出ERROR错误日志信息 int INFO Priority constant for the println method; use Log.i. 输出INFO程序日志信息 int VERBOSE Priority constant for the println method; use Log.v. 输出VERBOSE冗余日志信息

int WARN Priority constant for the println method; use Log.w. 输出WARN警告日志信息

参考示例：

package com.ceac.test7;import android.support.v7.app.ActionBarActivity;import android.os.Bundle;import android.util.Log;import android.view.Menu;import android.view.MenuItem;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class MainActivity extends ActionBarActivity implements OnClickListener{	private Button button;	private Button button2;	private Button button3;	private Button button4;	private Button button5;	    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);                button = (Button) findViewById(R.id.button1);        button2 = (Button) findViewById(R.id.button2);        button3 = (Button) findViewById(R.id.button3);        button4 = (Button) findViewById(R.id.button4);        button5 = (Button) findViewById(R.id.button5);                button.setOnClickListener(this);        button2.setOnClickListener(this);        button3.setOnClickListener(this);        button4.setOnClickListener(this);        button5.setOnClickListener(this);            }	@Override	public void onClick(View v) {		switch (v.getId()) {		case R.id.button1:			Log.d("debug", "DEBUG");			break;		case R.id.button2:			Log.e("error", "ERROR");			break;		case R.id.button3:			Log.i("info", "INFO");			break;		case R.id.button4:			Log.v("verbose", "VERBOSE");			break;		case R.id.button5:			Log.w("warn", "WARN");			break;		default:			break;		}			}}

运行结果：

Android源码详解：

/* * 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;import java.net.UnknownHostException;/** * API for sending log output. * * 
Generally, use the Log.v() Log.d() Log.i() Log.w() and Log.e() * methods. * * 
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. * * 
Tip: A good convention is to declare a TAG constant * in your class: * * 
private static final String TAG = "MyActivity";
 * * and use that in subsequent calls to the log methods. * 
 * * 
Tip: Don't forget that when you make a call like * 
Log.v(TAG, "index=" + i);
 * 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); }    }    /**     * Interface to handle terrible failures from {@link #wtf()}.     *     * @hide     */    public interface TerribleFailureHandler {        void onTerribleFailure(String tag, TerribleFailure what);    }    private static TerribleFailureHandler sWtfHandler = new TerribleFailureHandler() {            public void onTerribleFailure(String tag, TerribleFailure what) {                RuntimeInit.wtf(tag, what);            }        };    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.
    
    
     '     *  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.
     
      =
      
       '     *  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) {        TerribleFailure what = new TerribleFailure(msg, tr);        int bytes = println_native(LOG_ID_MAIN, ASSERT, tag, msg + '\n' + getStackTraceString(tr));        sWtfHandler.onTerribleFailure(tag, what);        return bytes;    }    /**     * Sets the terrible failure handler, for testing.     *     * @return the old handler     *     * @hide     */    public static TerribleFailureHandler setWtfHandler(TerribleFailureHandler handler) {        if (handler == null) {            throw new NullPointerException("handler == null");        }        TerribleFailureHandler oldHandler = sWtfHandler;        sWtfHandler = handler;        return oldHandler;    }    /**     * 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 "";        }        // This is to reduce the amount of log spew that apps do in the non-error        // condition of the network being unavailable.        Throwable t = tr;        while (t != null) {            if (t instanceof UnknownHostException) {                return "";            }            t = t.getCause();        }        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);}
      
     
    
   

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