Android中数据的保存

1. 概述

在Android中有以下几种保存数据方式：

1). 使用sharedPreference去保存:只有应用程序自己可以访问

2). 保存在应用程序私有的文件夹下：只有应用程序自己可以访问

3). 使用File形式保存在file/cache目录下

4). 保存到公共的sd卡上:其他的应用程序也可以访问

5). 使用数据库去保存（MySQL）

其中1.2.3的共性都是保存在当前应用的目录下的私有数据

2. 方法一：File方式保存

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        try{
            File f = new File("/data/data/com.example.aplex.testforgit/test.txt");
            BufferedWriter writer = new BufferedWriter(new FileWriter(f)) ;
            writer.write("hahahehe");
            writer.close();
        }catch (IOException e){
            e.printStackTrace();
        }

    }

3. 方法二：File方式保存（data/cache目录下）

protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        try{
            //1. 保存在应用的cache中
            {
                File f = new File(getCacheDir(),"cache.txt");
                BufferedWriter writer = new BufferedWriter(new FileWriter(f)) ;
                writer.write("hahahehe");
                writer.close();
            }

        }catch (IOException e){
            e.printStackTrace();
        }
        //2. 保存在应用的file中
            try {
                File f = new File(getFilesDir(),"file.txt");
                BufferedWriter writer = new BufferedWriter(new FileWriter(f)) ;
                writer.write("hahahehe");
                writer.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
    }

注意：清除数据会把data目录下改应用的所有数据都清除掉，而清除缓存则只会清除cache目录下的数据

4. 方法三：使用sharedPreference去保存

Xml 文件可以用来保存数据

注意: 使用sharedPreferences保存数据时,  生成的xml 文件是

在  /data/data/包名/shared_prefs 目录下.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:orientation="vertical"
    tools:context="com.example.aplex_new1.myapplication.MainActivity">

    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="请输入账号"
        android:inputType="number"
        android:id="@+id/ed"/>
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="清输入密码："
        android:inputType="numberPassword"
        android:id="@+id/ed2"/>
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="确认"
        android:id="@+id/bt" />

</LinearLayout>

package com.example.aplex_new1.myapplication;

import android.content.SharedPreferences;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {
    Button bt1;
    EditText accountView;
    EditText pwdView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        bt1 = findViewById(R.id.bt);
        accountView = findViewById(R.id.ed);
        pwdView = findViewById(R.id.ed2);
        //获取到mydata.xml对象，没有则会创建一个
        SharedPreferences sp = getSharedPreferences("mydata", 0);
        //拿到编辑器
        SharedPreferences.Editor ed = sp.edit();
        //获取出mydata.xml中键值为account的数据
        String account = sp.getString("account", "");
        //获取出mydata.xml中键值为pwd的数据
        String pwd = sp.getString("pwd","");

        //显示出来
        accountView.setText(account);
        pwdView.setText(pwd);

        bt1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                SharedPreferences sp = getSharedPreferences("mydata", 0);
                SharedPreferences.Editor ed = sp.edit();

                String account = accountView.getText().toString();
                String pwd = pwdView.getText().toString();
                //放入数据中
                ed.putString("account", account);
                ed.putString("pwd", pwd);
                //提交
                ed.apply();
            }
        });
    }
}

现象：第一次点击应用

账号密码都为空

在输入完账号密码后，退出，再次点击，则会回显出来

因为数据已经保存进了XML文件中去了

打开mydata.xml

<?xml version='1.0' encoding='utf-8' standalone='yes' ?>
<map>
    <string name="account">1234567</string>
    <string name="pwd">1234567</string>
</map>

4. 方法四：保存到公共的sd卡上:

1. 概念和代码

 AndroidManifest.xml

<!-- 在SDCard中创建与删除文件权限 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />
<!-- 往SDCard写入数据权限 -->
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

<application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
</application>

SDFileHelper.java

public class SDFileHelper {
    String TAG = "SDFileHelper";

    private Context context;

    public SDFileHelper() {
    }

    public SDFileHelper(Context context) {
        super();
        this.context = context;
    }

    //往SD卡写入文件的方法
    public void savaFileToSD(String filename, String filecontent) throws Exception {
        //如果手机已插入sd卡,且app具有读写sd卡的权限
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            Log.d(TAG,"文件名："+filename);
            //这里就不要用openFileOutput了,那个是往手机内存中写数据的
            FileOutputStream output = new FileOutputStream(filename);

            output.write(filecontent.getBytes());
            //将String字符串以字节流的形式写入到输出流中
            output.close();
            //关闭输出流
        } else Toast.makeText(context, "SD卡不存在或者不可读写", Toast.LENGTH_SHORT).show();
    }

    //读取SD卡中文件的方法
    //定义读取文件的方法:
    public String readFromSD(String filename) throws IOException {
        StringBuilder sb = new StringBuilder("");
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            //打开文件输入流
            FileInputStream input = new FileInputStream(filename);
            byte[] temp = new byte[];

            int len = ;
            //读取文件内容:
            while ((len = input.read(temp)) > ) {
                sb.append(new String(temp, , len));
            }
            //关闭输入流
            input.close();
        }
        return sb.toString();
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

    private EditText editname;
    private EditText editdetail;
    private Button btnsave;
    private Button btnclean;
    private Button btnread;
    private Context mContext;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mContext = getApplicationContext();
        bindViews();
    }

    private void bindViews() {
        editname = (EditText) findViewById(R.id.edittitle);
        editdetail = (EditText) findViewById(R.id.editdetail);
        btnsave = (Button) findViewById(R.id.btnsave);
        btnclean = (Button) findViewById(R.id.btnclean);
        btnread = (Button) findViewById(R.id.btnread);

        btnsave.setOnClickListener(this);
        btnclean.setOnClickListener(this);
        btnread.setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.btnclean:
                editdetail.setText("");
                editname.setText("");
                break;
            case R.id.btnsave:
                String filename = editname.getText().toString();
                String filedetail = editdetail.getText().toString();
                SDFileHelper sdHelper = new SDFileHelper(mContext);
                try
                {
                    sdHelper.savaFileToSD(filename, filedetail);
                    Toast.makeText(getApplicationContext(), "数据写入成功", Toast.LENGTH_SHORT).show();
                }
                catch(Exception e){
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "数据写入失败", Toast.LENGTH_SHORT).show();
                }
                break;
            case R.id.btnread:
                String detail = "";
                SDFileHelper sdHelper2 = new SDFileHelper(mContext);
                try
                {
                    String filename2 = editname.getText().toString();
                    detail = sdHelper2.readFromSD(filename2);
                }
                catch(IOException e){e.printStackTrace();}
                Toast.makeText(getApplicationContext(), detail, Toast.LENGTH_SHORT).show();
                break;
        }
    }
}

2. 6.0上SD卡的权限和动态申请权限的问题（重点！！）

Android6.0之后系统对权限的管理更加严格了，不但要在AndroidManifest中添加，还要在应用运行的时候动态申请。下面是动态申请SD卡读写的权限

1. 在AndroidManifest中添加SD卡读写的权限

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

2. 申请权限

申请权限有两种方式：

1). 手动打开权限：

Settings-->Apps-->找到所需要的应用-->Permissions-->打开权限

2). 在程序中动态申请权限（重点）：

 private static final int REQUEST_EXTERNAL_STORAGE = ;
    private static String[] PERMISSIONS_STORAGE = {
            "android.permission.READ_EXTERNAL_STORAGE",
            "android.permission.WRITE_EXTERNAL_STORAGE" };

    public static void verifyStoragePermissions(Activity activity) {

        try {
        //检测是否有写的权限
            int permission = ActivityCompat.checkSelfPermission(activity,
                    "android.permission.WRITE_EXTERNAL_STORAGE");
            if (permission != PackageManager.PERMISSION_GRANTED) {
        // 没有写的权限，去申请写的权限，会弹出对话框
                ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

5. 方法五：使用数据库（MySQL）去保存

MySQL数据库

SQLite数据库