有些情况下，不方便使用断点的方式来调试，而是希望在控制台打印输出日志，使用过Eclipse的同学都知道Java可以使用 System.out.println(""); 来在控制台打印输出日志，但是在android studio中却是不行的，还是有差别的，那应该用什么呢？
android.util.Log
在调试代码的时候我们需要查看调试信息，那我们就需要用Android Log类。
android.util.Log常用的方法有以下5个：Log.v() Log.d() Log.i() Log.w() 以及 Log.e() 。根据首字母对应VERBOSE，DEBUG,INFO, WARN，ERROR。
1、Log.v 的调试颜色为黑色的，任何消息都会输出，这里的v代表verbose啰嗦的意思，平时使用就是Log.v("","");
2、Log.d的输出颜色是蓝色的，仅输出debug调试的意思，但他会输出上层的信息，过滤起来可以通过DDMS的Logcat标签来选择.
3、Log.i的输出为绿色，一般提示性的消息information，它不会输出Log.v和Log.d的信息，但会显示i、w和e的信息
4、Log.w的意思为橙色，可以看作为warning警告，一般需要我们注意优化Android代码，同时选择它后还会输出Log.e的信息。
5、Log.e为红色，可以想到error错误，这里仅显示红色的错误信息，这些错误就需要我们认真的分析，查看栈的信息了。
注意：不同的打印方法在使用时都是某个方法带上(String tag, String msg)参数，tag表示的是打印信息的标签，msg表示的是需要打印的信息。
Log.java类
/*
* 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 java.io.PrintWriter;
import java.io.StringWriter;
import java.net.UnknownHostException;
/**
* Mock Log implementation for testing on non android host.
*/
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;
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(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(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(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(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(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(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(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(LOG_ID_MAIN, WARN, tag, msg + '\n' + getStackTraceString(tr));
}
/*
* 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(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(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(LOG_ID_MAIN, ERROR, tag, msg + '\n' + getStackTraceString(tr));
}
/**
* 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);
pw.flush();
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(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 final int LOG_ID_CRASH = 4;
/** @hide */ @SuppressWarnings("unused")
public static int println(int bufID,
int priority, String tag, String msg) {
return 0;
}
}
有小伙伴问我怎么在Android Studio中查看Log类？
在代码编辑器中，将光标定位在Log上，然后按下快捷键：Ctrl+B，就可以打开类文件了。
Logcat窗口
我还真不知道这个窗口能不能完全关闭掉，它好像是和Android Monitor 是一体的。如下图：
我们可以在logcat上按下然后拖动鼠标，把它拉出来，弹出一个单独的窗口。
关闭窗口之后，它又会回到Android Monitor 上。
上图中点击Restore ‘logcat’View 之后又弹出窗口，而不是和Android Monitor并排在标签上；
这时你可以在 ‘logcat’标签上按钮鼠标左键拖动它到Monitor标签旁边松开鼠标，它就回去了。
你也可以通过下拉列表框中的选项，或右侧的搜索框输入指定的关键词来筛选日志内容。
也可以通过右侧的编辑筛选配置来创建特定的筛选类型，方便以后使用。
小结
关于日志的使用就说这么多。
通过打印输出日志来调试是一种方法，却不是一种万能的方法。
为什么这么说呢？
因为有些时候（比如线程、网络操作之类的），打印日志的时候没有Bug，但把日志关了就会出Bug。
有些情况是调试模式运行的时候不出Bug，而正式运行的时候就出Bug。
系列导航：【Android 开发入门】
===========文后小料============
真正改变命运的其实并不是知识，而是这些知识带给你的能力的提高。也就是说，转化为能力的知识，才能够改变你的命运。
当你去追求一个百分之百的安全感的时候，你可能就只能把自己困在原地，哪儿都去不了，其实这是最不安全的。
今天这一代的员工，一定是通过提升自己的能力换来自己的安全。我们也更倾向于把价值存在朋友圈里边。
===========文档信息============
版权声明：非商用自由转载-保持署名-注明出处
署名(BY) ：testcs_dn(微wx笑)
文章出处：[无知人生，记录点滴](http://blog.csdn.net/testcs_dn)
