Android 的日志工具Log logcat说明 使用
不管是什么语言，不管是开发什么项目，日志是必须的。Android 中的日志工具类是Log（android.util.Log），这个类中提供了如下5 个方法来供我们打印日志。
Android中的日志，与java中的并无二致，也都是这样几个级别，无外乎实现不同而已。
### 分类
#### `Log.v()`
用于打印那些最为琐碎的、意义最小的日志信息。对应级别verbose ，是Android 日志里面级别最低的一种。
#### `Log.d()`
用于打印一些调试信息，这些信息对你调试程序和分析问题应该是有帮助的。
对应级别debug ，比verbose 高一级。
#### `Log.i()`
用于打印一些比较重要的数据，这些数据应该是你非常想看到的、可以帮你分析用户行为的数据。对应级别info ，比debug 高一级。
#### `Log.w()`
用于打印一些警告信息，提示程序在这个地方可能会有潜在的风险，最好去修复一下这些出现警告的地方。
对应级别warn ，比info 高一级。
#### `Log.e()`
用于打印程序中的错误信息，比如程序进入了catch语句中。
当有错误信息打印出来的时候，一般代表你的程序出现严重问题了，必须尽快修复。对应级别error ，比
warn 高一级。
当然，还有些他们自身的方法重载。
```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 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;
}
}
```
### tag
通过上面的log代码可以看出来，都有一个`String` 的参数，`tag`
这其实就是一个标记，写java的时候，经常可能会自己加一些字符串头，用来标记识别，这个tag就是这个作用。
### LogCat
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logcat提供了比较方面的检索工具的形式。
可以筛选模拟器；
可以筛选tag标签；
可以筛选日志级别；
可以自定义关键词、正则；
可以设置过滤器；
#### 过滤器
Show only selected application 表示只显示当前选中程序的日志；
Firebase 是Google 提供的一个开发者工具和基础架构平台；
No Filters 没有过滤器，会把所有的日志都显示出来；
还可以自定义过滤器，点击`Edit Filter Configuration`
!(data/attachment/forum/202207/28/204844cb323ashdxq2y09x.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/300 "image.png")
!(data/attachment/forum/202206/16/141330jha7st9soow8772i.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/300 "common_log.png")
`转载务必注明出处：程序员潇然，疯狂的字节X，https://crazybytex.com/thread-99-1-1.html `
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