java亟需关注的知识点-新I0（NIO）之大文件读取

java需要关注的知识点--新I0（NIO）之大文件读取
在读取大文件的时候，采用管道方式进行分流，使用byteBuffer把文件分成一段段的进行读写。
生成大文件 ：

public class ProductionFile {private static void productFile() throws FileNotFoundException {File file = new File("D://larger.txt");PrintWriter pw = new PrintWriter(file);try{for (int i = 0;i<1024;i++) {for (int j = 0;j <1024;j++){for(int k = 0;k<1024;i++){pw.write(i+":" +j);}pw.flush();}pw.flush();}}finally{pw.close();}}public static void main(String[] args) {try {productFile();} catch (FileNotFoundException e) {e.printStackTrace();}}}

public class LargeMappedFiles {static int length = 0x300000;/** * @param args * @throws IOException  * @throws FileNotFoundException  */public static void main(String[] args) throws FileNotFoundException, IOException {File file = new File("D://larger.txt");FileChannel fileChannel = new RandomAccessFile(file, "rw").getChannel();/** * map(FileChannel.MapMode mode,long position, long size) mode - * 根据是按只读、读取/写入或专用（写入时拷贝）来映射文件，分别为 FileChannel.MapMode 类中所定义的 * READ_ONLY、READ_WRITE 或 PRIVATE 之一 position - 文件中的位置，映射区域从此位置开始；必须为非负数 * size - 要映射的区域大小；必须为非负数且不大于 Integer.MAX_VALUE * 所以若想读取文件后半部分内容，如例子所写；若想读取文本后1 * /8内容，需要这样写map(FileChannel.MapMode.READ_ONLY, * f.length()*7/8,f.length()/8) * 想读取文件所有内容，需要这样写map(FileChannel.MapMode.READ_ONLY, 0,f.length()) */MappedByteBuffer inputBuffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, file.length()/40);long start = System.currentTimeMillis();byte[] dst = new byte[length];System.out.println("File size:" + inputBuffer.capacity());for( int offSet = 0; offSet<inputBuffer.capacity(); offSet += length) {if(inputBuffer.capacity() - offSet > length) {for (int i = 0 ; i < length ; i++) {dst[i] = inputBuffer.get(offSet + i);if (i == 10) System.out.println("i:" + i+ " Value-->"+new String(dst,0, 40)+ "   " );}}else {for (int i = 0 ; i< inputBuffer.capacity() - offSet; i++) {byte b = inputBuffer.get(offSet + i);}}}long end = System.currentTimeMillis(); System.out.println("Value-->"+new String(dst, 9990, 10000)+ "   " );System.out.println(end -start );//System.out.println(bb.get(count-10));}}

public class MappedIO {private static int numOfInts = 4000000;private static int numberOfUbuffInts = 200000;private abstract static class Tester{private String name;public Tester(String name) {this.name = name;}public void runTest() {System.out.print(name + ":");try{long start = System.nanoTime();test();double duration = System.nanoTime() - start;System.out.format("%.2f\n",duration/1.0e9);}catch(IOException e){throw new RuntimeException(e);}}public abstract void test() throws IOException;}private static Tester[] tests = {new Tester("Stream Write") {public void test() throws IOException {DataOutputStream dos = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(new File("temp.temp"))));for(int i = 0 ;i<numOfInts ;i++){dos.writeInt(i);}dos.close();}},new Tester("Mapped Write") {public void test() throws IOException {FileChannel fc = new RandomAccessFile("temp.temp", "rw").getChannel();IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size()).asIntBuffer();for(int i = 0; i<numOfInts; i++)ib.put(i);fc.close();}}, new Tester("Stream Reader") {public void test() throws IOException {DataInputStream dis = new DataInputStream(new BufferedInputStream(new FileInputStream(new File("temp.temp"))));for (int i = 0; i<numOfInts; i++) {dis.readInt();}dis.close();}} ,new Tester("Mapped Reader") {public void test() throws IOException {FileChannel fc = new RandomAccessFile("temp.temp","rw").getChannel();IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size()).asIntBuffer();while(ib.hasRemaining()) {ib.get();}fc.close();}},new Tester("Stream Writer Reader") {public void test() throws IOException {RandomAccessFile raf = new RandomAccessFile(new File("temp.temp"),"rw");raf.writeInt(1);for(int i = 0; i<numberOfUbuffInts; i++){raf.seek(raf.length()-4);raf.writeInt(raf.readInt());}raf.close();}},new Tester("Mapped Writer/Reader") {public void test() throws IOException {FileChannel fc = new RandomAccessFile(new File("temp.temp"), "rw").getChannel();IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size()).asIntBuffer();ib.put(0);for(int i=1;i<numberOfUbuffInts;i++) {ib.put(ib.get(i-1));}fc.close();}}};public static void main(String args[]) throws IOException {for (Tester test:tests) {test.runTest();}}}