本文概述

Android外部存储空间是我们执行读写操作的存储空间。外部存储器中的文件存储在/ sdcard或/ storage文件夹等中。保存在外部存储器中的文件是可读的, 并且可由用户修改。

在访问应用程序外部存储中的文件之前, 我们应检查设备中外部存储的可用性。

写入外部存储中的文件

java.io包提供openFileOutput()方法, 该方法返回FileOutputStream类的实例, 以将文件写入设备的外部存储中。通过调用getExternalFilesDir()获取仅由你的应用程序使用的目录。要将数据写入文件, 请调用FileOutputStream .write()方法。

var myExternalFile:File = File(getExternalFilesDir(filepath), fileName)

try {

val fileOutPutStream = FileOutputStream(myExternalFile)

fileOutPutStream.write(fileContent.toByteArray())

fileOutPutStream.close()

} catch (e: IOException) {

e.printStackTrace()

}

从外部存储读取文件内容

java.io包提供了openFileInput()方法, 该方法返回FileInputStream类的实例, 并从设备的外部存储读取文件。要从文件读取数据, 请调用BufferedReader()。readLine()。

var myExternalFile:File = File(getExternalFilesDir(filepath), fileName)

val filename = fileName.text.toString()

myExternalFile = File(getExternalFilesDir(filepath), filename)

var fileInputStream =FileInputStream(myExternalFile)

var inputStreamReader: InputStreamReader = InputStreamReader(fileInputStream)

val bufferedReader: BufferedReader = BufferedReader(inputStreamReader)

val stringBuilder: StringBuilder = StringBuilder()

var text: String? = null

while ({ text = bufferedReader.readLine(); text }() != null) {

stringBuilder.append(text)

}

fileInputStream.close()

外部存储访问权限

在AndroidManifest.xml文件中添加以下权限。

Kotlin Android读写外部存储示例

在此示例中, 我们将数据写入外部存储设备中的文件, 并从同一外部存储设备读取相同的文件内容。

activity_main.xml

在activity_main.xml文件中添加以下代码。在此文件中, 为输入文件名, 文件内容添加两个EditText, 并为保存和查看文件内容添加两个Button。

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="example.srcmini.com.kotlinexternalstoragereadwrite.MainActivity">

android:id="@+id/textView"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignLeft="@+id/textView2"

android:layout_alignParentTop="true"

android:layout_alignStart="@+id/textView2"

android:layout_marginTop="68dp"

android:text="File Name"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"

app:layout_constraintBottom_toBottomOf="parent"

app:layout_constraintEnd_toEndOf="parent"

app:layout_constraintHorizontal_bias="0.027"

app:layout_constraintStart_toStartOf="parent"

app:layout_constraintTop_toTopOf="parent"

app:layout_constraintVertical_bias="0.065" />

android:id="@+id/textView2"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignBottom="@+id/editTextData"

android:layout_alignParentLeft="true"

android:layout_alignParentStart="true"

android:layout_marginBottom="36dp"

android:layout_marginLeft="50dp"

android:layout_marginStart="50dp"

android:text="File Data"

android:textAppearance="@style/Base.TextAppearance.AppCompat.Medium"

app:layout_constraintBottom_toBottomOf="parent"

app:layout_constraintEnd_toEndOf="parent"

app:layout_constraintHorizontal_bias="0.027"

app:layout_constraintStart_toStartOf="parent"

app:layout_constraintTop_toBottomOf="@+id/textView"

app:layout_constraintVertical_bias="0.167" />

android:id="@+id/editTextFile"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignLeft="@+id/editTextData"

android:layout_alignStart="@+id/editTextData"

android:layout_alignTop="@+id/textView"

android:ems="10"

android:inputType="none" />

android:id="@+id/editTextData"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignParentEnd="true"

android:layout_alignParentRight="true"

android:layout_below="@+id/editTextFile"

android:layout_marginEnd="37dp"

android:layout_marginRight="37dp"

android:layout_marginTop="30dp"

android:ems="10"

android:inputType="none"

android:lines="5" />

android:id="@+id/button_save"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignParentBottom="true"

android:layout_marginBottom="68dp"

android:layout_toLeftOf="@+id/editTextData"

android:layout_toStartOf="@+id/editTextData"

android:text="Save" />

android:id="@+id/button_view"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:layout_alignBottom="@+id/button_save"

android:layout_alignEnd="@+id/editTextData"

android:layout_alignRight="@+id/editTextData"

android:layout_marginEnd="43dp"

android:layout_marginRight="43dp"

android:text="View" />

MainActivity.kt

在MainActivity.kt类中添加以下代码。在此类中, 我们将通过单击保存按钮将文件名和数据保存在外部存储器中, 并通过单击查看按钮来检索文件内容。

package example.srcmini.com.kotlinexternalstoragereadwrite

import android.support.v7.app.AppCompatActivity

import android.os.Bundle

import android.view.View

import android.widget.Button

import android.widget.EditText

import android.widget.Toast

import android.os.Environment

import java.io.*

class MainActivity : AppCompatActivity() {

private val filepath = "MyFileStorage"

internal var myExternalFile: File?=null

private val isExternalStorageReadOnly: Boolean get() {

val extStorageState = Environment.getExternalStorageState()

return if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(extStorageState)) {

true

} else {

false

}

}

private val isExternalStorageAvailable: Boolean get() {

val extStorageState = Environment.getExternalStorageState()

return if (Environment.MEDIA_MOUNTED.equals(extStorageState)) {

true

} else{

false

}

}

override fun onCreate(savedInstanceState: Bundle?) {

super.onCreate(savedInstanceState)

setContentView(R.layout.activity_main)

val fileName = findViewById(R.id.editTextFile) as EditText

val fileData = findViewById(R.id.editTextData) as EditText

val saveButton = findViewById(R.id.button_save) as Button

val viewButton = findViewById(R.id.button_view) as Button

saveButton.setOnClickListener(View.OnClickListener {

myExternalFile = File(getExternalFilesDir(filepath), fileName.text.toString())

try {

val fileOutPutStream = FileOutputStream(myExternalFile)

fileOutPutStream.write(fileData.text.toString().toByteArray())

fileOutPutStream.close()

} catch (e: IOException) {

e.printStackTrace()

}

Toast.makeText(applicationContext, "data save", Toast.LENGTH_SHORT).show()

})

viewButton.setOnClickListener(View.OnClickListener {

myExternalFile = File(getExternalFilesDir(filepath), fileName.text.toString())

val filename = fileName.text.toString()

myExternalFile = File(getExternalFilesDir(filepath), filename)

if(filename.toString()!=null && filename.toString().trim()!=""){

var fileInputStream =FileInputStream(myExternalFile)

var inputStreamReader: InputStreamReader = InputStreamReader(fileInputStream)

val bufferedReader: BufferedReader = BufferedReader(inputStreamReader)

val stringBuilder: StringBuilder = StringBuilder()

var text: String? = null

while ({ text = bufferedReader.readLine(); text }() != null) {

stringBuilder.append(text)

}

fileInputStream.close()

//Displaying data on EditText

Toast.makeText(applicationContext, stringBuilder.toString(), Toast.LENGTH_SHORT).show()

}

})

if (!isExternalStorageAvailable || isExternalStorageReadOnly) {

saveButton.isEnabled = false

}

}

}

AndroidManifest.xml

package="example.srcmini.com.kotlinexternalstoragereadwrite">

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">

输出：