Save the log file to the sdcard, directory: crash folder under the sdcard root directory

First, set permissions, no permissions to the final effect

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
Two classes must be created to collect error information for exceptions
Here is the CrashHandler class

/**
 * Created by BAIPEI on 2017/12/5.
 */

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.io.Writer;
import java.lang.Thread.UncaughtExceptionHandler;
import java.lang.reflect.Field;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import android.content.Context;
import android.content.pm.PackageInfo;
import android.content.pm.PackageManager;
import android.content.pm.PackageManager.NameNotFoundException;
import android.os.Build;
import android.os.Environment;
import android.os.Looper;
import android.util.Log;
import android.widget.Toast;

public class CrashHandler implements UncaughtExceptionHandler{
    private static final String TAG = "CrashHandler";
    private Thread.UncaughtExceptionHandler mDefaultHandler;// System default UncaughtException processing class
    private static CrashHandler INSTANCE = new CrashHandler();// CrashHandler instance
    private Context mContext;// Context object for program
    private Map<String, String> info = new HashMap<String, String>();// Used to store device information and exception information
    private SimpleDateFormat format = new SimpleDateFormat(
            "yyyy-MM-dd-HH-mm-ss");// Used to format the date as part of the log file name

    /** Make sure there is only one instance of CrashHandler */
    private CrashHandler() {

    }

    /** Get CrashHandler instance, singleton mode */
    public static CrashHandler getInstance() {
        return INSTANCE;
    }

    /**
     * Initialization
     *
     * @param context
     */
    public void init(Context context) {
        mContext = context;
        mDefaultHandler = Thread.getDefaultUncaughtExceptionHandler();// Get the system default UncaughtException processor
        Thread.setDefaultUncaughtExceptionHandler(this);// Set this CrashHandler as the program's default processor
    }
    /**
     * When UncaughtException occurs, it goes into the overridden method to handle it
     */
    public void uncaughtException(Thread thread, Throwable ex) {
        if (!handleException(ex) && mDefaultHandler != null) {
            // Let the default exception handler of the system handle the custom unhandled
            mDefaultHandler.uncaughtException(thread, ex);
        } else {
            try {
                Thread.sleep(3000);// If processed, allow the program to continue running for 3 seconds before exiting, ensuring that the file is saved and uploaded to the server
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // Exit the program
            android.os.Process.killProcess(android.os.Process.myPid());
            System.exit(1);
        }
    }

    /**
     * Customize error handling, collect error information and send error reports.
     *
     * @param ex
     *            Exception Information
     * @return true If the exception information is handled; otherwise, false is returned.
     */
    public boolean handleException(Throwable ex) {
        if (ex == null)
            return false;
        new Thread() {
            public void run() {
                Looper.prepare();
                Toast.makeText(mContext, "I'm sorry,Program Exception,About to quit", Toast.LENGTH_SHORT).show();
                Looper.loop();
            }
        }.start();
        // Collect device parameter information
        collectDeviceInfo(mContext);
        // Save Log File
        saveCrashInfo2File(ex);
        return true;
    }

    /**
     * Collect device parameter information
     *
     * @param context
     */
    public void collectDeviceInfo(Context context) {
        try {
            PackageManager pm = context.getPackageManager();// Get Package Manager
            PackageInfo pi = pm.getPackageInfo(context.getPackageName(),
                    PackageManager.GET_ACTIVITIES);// Get information about the app, the main Activity
            if (pi != null) {
                String versionName = pi.versionName == null ? "null"
                        : pi.versionName;
                String versionCode = pi.versionCode + "";
                info.put("versionName", versionName);
                info.put("versionCode", versionCode);
            }
        } catch (NameNotFoundException e) {
            e.printStackTrace();
        }

        Field[] fields = Build.class.getDeclaredFields();// Reflection mechanism
        for (Field field : fields) {
            try {
                field.setAccessible(true);
                info.put(field.getName(), field.get("").toString());
                Log.d(TAG, field.getName() + ":" + field.get(""));
            } catch (IllegalArgumentException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
    }
    private String saveCrashInfo2File(Throwable ex) {
        StringBuffer sb = new StringBuffer();
        for (Map.Entry<String, String> entry : info.entrySet()) {
            String key = entry.getKey();
            String value = entry.getValue();
            sb.append(key + "=" + value + "\r\n");
        }
        Writer writer = new StringWriter();
        PrintWriter pw = new PrintWriter(writer);
        ex.printStackTrace(pw);
        Throwable cause = ex.getCause();
        // Loop to write all exception information to the writer
        while (cause != null) {
            cause.printStackTrace(pw);
            cause = cause.getCause();
        }
        pw.close();// Remember to close
        String result = writer.toString();
        sb.append(result);
        // Save File
        long timetamp = System.currentTimeMillis();
        String time = format.format(new Date());
        String fileName = "crash-" + time + "-" + timetamp + ".log";
        if (Environment.getExternalStorageState().equals(
                Environment.MEDIA_MOUNTED)) {
            try {
                File dir = new File(Environment.getExternalStorageDirectory().getAbsolutePath() +                           File.separator + "crash");
                Log.i("CrashHandler", dir.toString());
                if (!dir.exists())
                    dir.mkdir();
                FileOutputStream fos = new FileOutputStream(new File(dir,
                        fileName));
                fos.write(sb.toString().getBytes());
                fos.close();
                return fileName;
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return null;
    }
}
Here is the CrashApplication class

import android.app.Application;

/**
 * Created by BAIPEI on 2017/12/5.
 */

public class CrashApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        CrashHandler crashHandler = CrashHandler.getInstance();
        crashHandler.init(this);
    }
}
Here's MainActivity, because we want to make an exception, if we don't call that exception here,
That exception won't work, and we won't get the results we want
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;


public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button mBtn;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();

    }

    private void initView() {
        mBtn = (Button) findViewById(R.id.btn);
        mBtn.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        //Call this exception in a click event
        throw new RuntimeException("String");
    }
}
Here is the layout file, which is also a click event
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="bwie.com.crashhandler_diqitian.MainActivity">

    <Button
        android:id="@+id/btn"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="click"/>

</android.support.constraint.ConstraintLayout>
Don't forget to put AndroidManifest.xml Change the application inside
<application android:name=".CrashApplication"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity android:name=".MainActivity">
            ...
        </activity>
    </application>