在Android中，可以使用Logcat来输出日志信息。然而，如果想将日志写入文件，可以使用第三方库来实现。下面是使用Timber库将日志写入文件的示例代码：

1. 首先，在你的项目的build.gradle文件中添加以下依赖项：

implementation 'com.jakewharton.timber:timber:4.7.1'
implementation 'com.squareup.okio:okio:2.10.0'

2. 创建一个自定义的Application类，并在onCreate()方法中进行初始化：

import android.app.Application;
import timber.log.Timber;

public class MyApp extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        
        // 初始化Timber
        if (BuildConfig.DEBUG) {
            Timber.plant(new Timber.DebugTree());
        } else {
            Timber.plant(new FileLoggingTree(getApplicationContext()));
        }
    }
}

3. 创建一个自定义的FileLoggingTree类，用于将日志写入文件：

import android.content.Context;
import android.os.Environment;
import android.util.Log;

import org.jetbrains.annotations.NotNull;

import java.io.BufferedWriter;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Locale;

import timber.log.Timber;

public class FileLoggingTree extends Timber.DebugTree {
    private static final String LOG_TAG = "FileLoggingTree";
    private static final String LOG_FILE_NAME = "log.txt";
    private static final String LOG_FOLDER_NAME = "MyAppLogs";
    
    private final Context context;
    
    public FileLoggingTree(Context context) {
        this.context = context;
    }
    
    @Override
    protected void log(int priority, String tag, @NotNull String message, Throwable t) {
        if (!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            // External storage is not available
            return;
        }
        
        File logFile = getLogFile();
        
        try {
            BufferedWriter writer = new BufferedWriter(new FileWriter(logFile, true));
            writer.write(getFormattedLogMessage(priority, tag, message));
            writer.newLine();
            writer.flush();
            writer.close();
        } catch (IOException e) {
            Log.e(LOG_TAG, "Error writing log message to file", e);
        }
    }
    
    private File getLogFile() {
        File logFolder = new File(context.getExternalFilesDir(null), LOG_FOLDER_NAME);
        if (!logFolder.exists()) {
            logFolder.mkdirs();
        }
        
        return new File(logFolder, LOG_FILE_NAME);
    }
    
    private String getFormattedLogMessage(int priority, String tag, String message) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("MM-dd HH:mm:ss.SSS", Locale.getDefault());
        String timestamp = dateFormat.format(new Date());
        return String.format(Locale.getDefault(), "%s %s/%s: %s", timestamp, getPriorityString(priority), tag, message);
    }
    
    private String getPriorityString(int priority) {
        switch (priority) {
            case Log.VERBOSE:
                return "V";
            case Log.DEBUG:
                return "D";
            case Log.INFO:
                return "I";
            case Log.WARN:
                return "W";
            case Log.ERROR:
                return "E";
            case Log.ASSERT:
                return "A";
            default:
                return "U";
        }
    }
}

4. 在AndroidManifest.xml文件中将自定义的Application类设置为应用的默认Application：

    ...

现在，当你使用Timber的d(), e(), i(), v(), w()或wtf()方法输出日志时，日志信息将被写入到外部存储设备中的MyAppLogs/log.txt文件中。请注意，为了使日志输出到文件中，需要授予应用的存储权限。