Android开发笔记（三十三）文本文件和图片文件的读写

文本文件读写

简单文件读写一般是借助于FileOutputStream和FileInputStream，其中FileOutputStream用于写文件，而FileInputStream用于读文件。
写文件时，先把File文件转为FileOutputStream对象，然后调用该对象的write方法写入文件。注意write方法的参数是字节流byte[]，所以需要把要保存的文本串转为字节流后才能操作。
读文件时，也是先把File文件转为FileInputStream对象，然后调用该对象的available方法获得字节流的长度，分配指定长度的临时字节流；接着把文件内容读入这个临时字节流，最后把临时字节流转换为String字符串。


下面是读写文本文件的代码片段：

	private void saveTxtFile(String path, String txt) {
		try {
			FileOutputStream fos = new FileOutputStream(path);
			fos.write(txt.getBytes());
			fos.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}


	private String openTxtFile(String path) {
		String readStr = "";
		try {
			FileInputStream fis = new FileInputStream(path);
			byte[] b = new byte[fis.available()];
			fis.read(b);
			readStr = new String(b);
		} catch (Exception e) {
			e.printStackTrace();
		}
		return readStr;
	}

图片文件读写

Android上的图片处理类是Bitmap，代码中读写Bitmap也可以使用FileOutputStream和FileInputStream。不过在频繁读写图片时，这两个类的处理性能不如BufferedOutputStream和BufferedInputStream。所以在实际编码中，读写图片文件一般还是用BufferedOutputStream和BufferedInputStream。
保存图片文件时，调用Bitmap的compress方法，指定图片类型、压缩质量，以及图片的BufferedOutputStream对象，然后调用BufferedOutputStream对象的flush方法完成写入。
打开图片文件时，使用BitmapFactory的decodeStream方法，从BufferedInputStream对象中读取Bitmap图片信息。


下面是读写图片文件的代码片段：

	private void saveBitmap(String path, Bitmap bitmap) {
		try {
			BufferedOutputStream bos = new BufferedOutputStream(
					new FileOutputStream(path));
			bitmap.compress(Bitmap.CompressFormat.JPEG, 80, bos);
			bos.flush();
			bos.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	
	private Bitmap openBitmap(String path) {
		Bitmap bitmap = null;
		try {
			BufferedInputStream bis = new BufferedInputStream(
					new FileInputStream(path));
			bitmap = BitmapFactory.decodeStream(bis);
			bis.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
		return bitmap;
	}

下面是一个完整的文件读写代码例子，包括文本文件读写和图片文件读写：

import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.text.SimpleDateFormat;
import java.util.Date;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

public class MainActivity extends Activity implements OnClickListener {
	
	private TextView tv_file;
	private ImageView iv_file;
	private String txtFile, imgFile;

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

		String externalDir = Environment.getExternalStorageDirectory().toString();
		txtFile = externalDir + "/a.txt";
		imgFile = externalDir + "/b.jpg";
		tv_file = (TextView) findViewById(R.id.tv_file);
		iv_file = (ImageView) findViewById(R.id.iv_file);
		tv_file.setDrawingCacheEnabled(true);
		
		Button btn_write = (Button) findViewById(R.id.btn_write);
		Button btn_read = (Button) findViewById(R.id.btn_read);
		btn_write.setOnClickListener(this);
		btn_read.setOnClickListener(this);
		Button btn_img_save = (Button) findViewById(R.id.btn_img_save);
		Button btn_img_open = (Button) findViewById(R.id.btn_img_open);
		btn_img_save.setOnClickListener(this);
		btn_img_open.setOnClickListener(this);
	}

	@Override
	public void onClick(View v) {
		if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)!=true) {
			Toast.makeText(this, "当前SD卡不可用", Toast.LENGTH_LONG).show();
			return;
		}
		if (v.getId() == R.id.btn_write) {
			String writeStr = "白日依山尽，\n黄河入海流。\n欲穷千里目，\n更上一层楼。";
			saveTxtFile(txtFile, writeStr);
			String writeDesc = String.format("当前时间为%s\n文件路径为%s\n", 
					getNowDateTime(), txtFile);
			tv_file.setText(writeDesc);
		} else if (v.getId() == R.id.btn_read) {
			File file = new File(txtFile);
			if (file.exists() != true) {
				Toast.makeText(this, txtFile+"文件不存在", Toast.LENGTH_LONG).show();
				return;
			}
			String readStr = openTxtFile(txtFile);
			String writeDesc = String.format("当前时间为%s\n文件路径为%s\n文件内容为：\n%s\n", 
					getNowDateTime(), txtFile, readStr);
			tv_file.setText(writeDesc);
		} else if (v.getId() == R.id.btn_img_save) {
			tv_file.setBackgroundColor(Color.WHITE);
			Bitmap bitmap = tv_file.getDrawingCache();
			saveBitmap(imgFile, bitmap);
		} else if (v.getId() == R.id.btn_img_open) {
			File file = new File(imgFile);
			if (file.exists() != true) {
				Toast.makeText(this, imgFile+"文件不存在", Toast.LENGTH_LONG).show();
				return;
			}
			Bitmap bitmap = openBitmap(imgFile);
			iv_file.setImageBitmap(bitmap);
		}
	}

	private String getNowDateTime() {
		SimpleDateFormat s_format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		Date d_date = new Date();
		String s_date = "";
		s_date = s_format.format(d_date);
		return s_date;
	}
	
	private void saveTxtFile(String path, String txt) {
		try {
			FileOutputStream fos = new FileOutputStream(path);
			fos.write(txt.getBytes());
			fos.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	private String openTxtFile(String path) {
		String readStr = "";
		try {
			FileInputStream fis = new FileInputStream(path);
			byte[] b = new byte[fis.available()];
			fis.read(b);
			fis.close();
			readStr = new String(b);
		} catch (Exception e) {
			e.printStackTrace();
		}
		return readStr;
	}
	
	private void saveBitmap(String path, Bitmap bitmap) {
		try {
			BufferedOutputStream bos = new BufferedOutputStream(
					new FileOutputStream(path));
			bitmap.compress(Bitmap.CompressFormat.JPEG, 80, bos);
			bos.flush();
			bos.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	
	private Bitmap openBitmap(String path) {
		Bitmap bitmap = null;
		try {
			BufferedInputStream bis = new BufferedInputStream(
					new FileInputStream(path));
			bitmap = BitmapFactory.decodeStream(bis);
			bis.close();
		} catch (Exception e) {
			e.printStackTrace();
		}
		return bitmap;
	}
}

点此查看Android开发笔记的完整目录