java.io和java.nio性能简单对比
我从java1.3开始学习java，后来主要用1.4，再后来1.5和1.6中的很多新特性，都停留在“知道”的状态，比如nio，虽然据说可以提升性能，但并没有真正深入使用和测试过，工作操作文件的情况不多，所以关注也不多，即便用到，也还是习惯性的用java.io。今天看到的这篇文章，虽然测试手段非常简单，所得结论也难免有些片面 ，但依然说明，在顺序访问的时候，NIO的性能相对java.io有很大的提升。也许应该update一下自己的知识了，否则就要OUT，或者早已经OUT了。
下次操作文件或者写socket要用NIO了。---
转自：http://links.techwebnewsletters.com/ctt?kn=28&m=34038811&r=MzI1Mjc3MDAzOAS2&b=0&j=NTc5NjM4MTAS1&mt=1&rt=0
以下为翻译的内容： 最近我在工作中用到了java i/o相关功能。因为对java.io的了解更多（毕竟面世较早），所以一开始我使用的是java.io包下的类，后来为了测试一下是不是能够通过NIO提高文件操作性能，于是转向了java.nio。我得到的结论让我感到有些震惊，下面是对比测试的一些细节：?? 1、在java.io的测试代码中，我使用RandomAccessFile直接向文件写数据，并搜索到特定的位置执行记录的插入、读取和删除。
?? 2、在java.nio的初步测试代码中，使用FileChannel对象。NIO之所以比java.io更加高效，是因为NIO面向的是data chunks，而java.io基本上是面向byte的。
?? 3、为了进一步挖掘NIO的能力，我又改用MappedByteBuffer执行测试，这个类是构建在操作系统的虚拟内存机制上的。根据java文档所说，这个类在性能方面是最好的。?为了进行测试，我写了一个模拟员工数据库的小程序，员工数据的结构如下：??? view plaincopy to clipboardprint?
class Employee {??
??????? String last; // the key??
??????? String first;??
??????? int id;??
??????? int zip;??
??????? boolean employed;??
??????? String comments;??
??? }?
class Employee {
??????? String last; // the key
??????? String first;
??????? int id;
??????? int zip;
??????? boolean employed;
??????? String comments;
??? } 员工数据写入文件，并将last name作为索引key，日后可以通过这个key从文件中加载该员工对应的数据。无论使用IO、NIO还是MappedByteBuffers，首先都需要打开一个RandomAccessFile。以下代码在用户的home目录下创建一个名为employee.ejb的文件，设置为可读可写，并初始化对应的Channel和MappedByteBuffer：
??? view plaincopy to clipboardprint?
String userHome = System.getProperty("user.home");??
??? StringBuffer pathname = new StringBuffer(userHome);??
??? pathname.append(File.separator);??
??? pathname.append("employees.ejb");??
??? java.io.RandomAccessFile journal =??
??????? new RandomAccessFile(pathname.toString(), "rw");??
???
??? //下面这一句是为了NIO??
??? java.nio.channels.FileChannel channel = journal.getChannel();??
?????
??? //下面这两句是为了使用MappedByteBuffer??
??? journal.setLength(PAGE_SIZE);??
??? MappedByteBuffer mbb =??
??????? channel.map(FileChannel.MapMode.READ_WRITE, 0, journal.length() );?
String userHome = System.getProperty("user.home");
??? StringBuffer pathname = new StringBuffer(userHome);
??? pathname.append(File.separator);
??? pathname.append("employees.ejb");
??? java.io.RandomAccessFile journal =
??????? new RandomAccessFile(pathname.toString(), "rw");
?
??? //下面这一句是为了NIO
??? java.nio.channels.FileChannel channel = journal.getChannel();
??
??? //下面这两句是为了使用MappedByteBuffer
??? journal.setLength(PAGE_SIZE);
??? MappedByteBuffer mbb =
??????? channel.map(FileChannel.MapMode.READ_WRITE, 0, journal.length() ); ?
使用channel.map进行映射后，当该文件被追加了新的数据时，之前的MappedByteBuffer是看不到这些数据的。因为我们想测试读和写，所以当文件中追加写入新的记录后，需要重新做映射才能使得MappedByteBuffer读取新数据。为了提高效率，降低重新映射的次数，每次空间不够的时候，我们将文件扩张特定的大小（比如说1K）以防止每次追加新记录都要重新映射。
?下面是写入员工记录的对比测试：使用java.io的代码：
??? view plaincopy to clipboardprint?
public boolean addRecord_IO(Employee emp) {??
??????? try {??
??????????? byte[] last = emp.last.getBytes();??
??????????? byte[] first = emp.first.getBytes();??
??????????? byte[] comments = emp.comments.getBytes();??
?????????????
??????????? // Just hard-code the sizes for perfomance??
??????????? int size = 0;??
??????????? size += emp.last.length();??
??????????? size += 4; // strlen - Integer??
??????????? size += emp.first.length();??
??????????? size += 4; // strlen - Integer??
??????????? size += 4; // emp.id - Integer??
??????????? size += 4; // emp.zip - Integer??
??????????? size += 1; // emp.employed - byte??
??????????? size += emp.comments.length();??
??????????? size += 4; // strlen - Integer??
??????????? long offset = getStorageLocation(size);??
??????????? //??
??????????? // Store the record by key and save the offset??
??????????? //??
??????????? if ( offset == -1 ) {??
??????????????? // We need to add to the end of the journal. Seek there??
??????????????? // now only if we're not already there??
??????????????? long currentPos = journal.getFilePointer();??
??????????????? long jounralLen = journal.length();??
??????????????? if ( jounralLen != currentPos )??
??????????????????? journal.seek(jounralLen);??
?????????????????????
??????????????? offset = jounralLen;??
??????????? }else {??
??????????????? // Seek to the returned insertion point??
??????????????? journal.seek(offset);??
??????????? }??
??????????? // Fist write the header??
??????????? journal.writeByte(1);??
??????????? journal.writeInt(size);??
??????????? // Next write the data??
??????????? journal.writeInt(last.length);??
??????????? journal.write(last);??
??????????? journal.writeInt(first.length);??
??????????? journal.write(first);??
??????????? journal.writeInt(emp.id);??
??????????? journal.writeInt(emp.zip);??
??????????? if ( emp.employed )??
??????????????? journal.writeByte(1);??
??????????? else?
??????????????? journal.writeByte(0);??
??????????? journal.writeInt(comments.length);??
??????????? journal.write(comments);??
??????????? // Next, see if we need to append an empty record if we inserted??
??????????? // this new record at an empty location??
??????????? if ( newEmptyRecordSize != -1 ) {??
??????????????? // Simply write a header??
??????????????? journal.writeByte(0); //inactive record??
??????????????? journal.writeLong(newEmptyRecordSize);??
??????????? }??
??????????? employeeIdx.put(emp.last, offset);??
??????????? return true;??
??????? }??
??????? catch ( Exception e ) {??
??????????? e.printStackTrace();??
??????? }??
??????? return false;??
??? }?
public boolean addRecord_IO(Employee emp) {
??????? try {
??????????? byte[] last = emp.last.getBytes();
??????????? byte[] first = emp.first.getBytes();
??????????? byte[] comments = emp.comments.getBytes();
??????????
??????????? // Just hard-code the sizes for perfomance
??????????? int size = 0;
??????????? size += emp.last.length();
??????????? size += 4; // strlen - Integer
??????????? size += emp.first.length();
??????????? size += 4; // strlen - Integer
??????????? size += 4; // emp.id - Integer
??????????? size += 4; // emp.zip - Integer
??????????? size += 1; // emp.employed - byte
??????????? size += emp.comments.length();
??????????? size += 4; // strlen - Integer
??????????? long offset = getStorageLocation(size);
??????????? //
??????????? // Store the record by key and save the offset
??????????? //
??????????? if ( offset == -1 ) {
??????????????? // We need to add to the end of the journal. Seek there
??????????????? // now only if we're not already there
??????????????? long currentPos = journal.getFilePointer();
??????????????? long jounralLen = journal.length();
??????????????? if ( jounralLen != currentPos )
??????????????????? journal.seek(jounralLen);
??????????????????
??????????????? offset = jounralLen;
??????????? }else {
??????????????? // Seek to the returned insertion point
??????????????? journal.seek(offset);
??????????? }
??????????? // Fist write the header
??????????? journal.writeByte(1);
??????????? journal.writeInt(size);
??????????? // Next write the data
??????????? journal.writeInt(last.length);
??????????? journal.write(last);
??????????? journal.writeInt(first.length);
??????????? journal.write(first);
??????????? journal.writeInt(emp.id);
??????????? journal.writeInt(emp.zip);
??????????? if ( emp.employed )
??????????????? journal.writeByte(1);
??????????? else
??????????????? journal.writeByte(0);
??????????? journal.writeInt(comments.length);
??????????? journal.write(comments);
??????????? // Next, see if we need to append an empty record if we inserted
??????????? // this new record at an empty location
??????????? if ( newEmptyRecordSize != -1 ) {
??????????????? // Simply write a header
??????????????? journal.writeByte(0); //inactive record
??????????????? journal.writeLong(newEmptyRecordSize);
??????????? }
??????????? employeeIdx.put(emp.last, offset);
??????????? return true;
??????? }
??????? catch ( Exception e ) {
??????????? e.printStackTrace();
??????? }
??????? return false;
??? }
?使用java.nio的代码：??? view plaincopy to clipboardprint?
public boolean addRecord_NIO(Employee emp) {??
??????? try {??
??????????? data.clear();??
??????????? byte[] last = emp.last.getBytes();??
??????????? byte[] first = emp.first.getBytes();??
??????????? byte[] comments = emp.comments.getBytes();??
??????????? data.putInt(last.length);??
??????????? data.put(last);??
??????????? data.putInt(first.length);??
??????????? data.put(first);??
??????????? data.putInt(emp.id);??
??????????? data.putInt(emp.zip);??
??????????? byte employed = 0;??
??????????? if ( emp.employed )??
??????????????? employed = 1;??
??????????? data.put(employed);??
??????????? data.putInt(comments.length);??
??????????? data.put(comments);??
??????????? data.flip();??
??????????? int dataLen = data.limit();??
??????????? header.clear();??
??????????? header.put((byte)1); // 1=active record??
??????????? header.putInt(dataLen);??
??????????? header.flip();??
??????????? long headerLen = header.limit();??
??????????? int length = (int)(headerLen + dataLen);??
??????????? long offset = getStorageLocation((int)dataLen);??
??????????? //??
??????????? // Store the record by key and save the offset??
??????????? //??
??????????? if ( offset == -1 ) {??
??????????????? // We need to add to the end of the journal. Seek there??
??????????????? // now only if we're not already there??
??????????????? long currentPos = channel.position();??
??????????????? long jounralLen = channel.size();??
??????????????? if ( jounralLen != currentPos )??
??????????????????? channel.position(jounralLen);??
??????????????? offset = jounralLen;??
??????????? }??
??????????? else {??
??????????????? // Seek to the returned insertion point??
??????????????? channel.position(offset);??
??????????? }??
??????????? // Fist write the header??
??????????? long written = channel.write(srcs);??
??????????? // Next, see if we need to append an empty record if we inserted??
??????????? // this new record at an empty location??
??????????? if ( newEmptyRecordSize != -1 ) {??
??????????????? // Simply write a header??
??????????????? data.clear();??
??????????????? data.put((byte)0);??
??????????????? data.putInt(newEmptyRecordSize);??
??????????????? data.flip();??
??????????????? channel.write(data);??
??????????? }??
??????????? employeeIdx.put(emp.last, offset);??
??????????? return true;??
??????? }??
??????? catch ( Exception e ) {??
??????????? e.printStackTrace();??
??????? }??
??????? return false;??
??? }?
public boolean addRecord_NIO(Employee emp) {
??????? try {
??????????? data.clear();
??????????? byte[] last = emp.last.getBytes();
??????????? byte[] first = emp.first.getBytes();
??????????? byte[] comments = emp.comments.getBytes();
??????????? data.putInt(last.length);
??????????? data.put(last);
??????????? data.putInt(first.length);
??????????? data.put(first);
??????????? data.putInt(emp.id);
??????????? data.putInt(emp.zip);
??????????? byte employed = 0;
??????????? if ( emp.employed )
??????????????? employed = 1;
??????????? data.put(employed);
??????????? data.putInt(comments.length);
??????????? data.put(comments);
??????????? data.flip();
??????????? int dataLen = data.limit();
??????????? header.clear();
??????????? header.put((byte)1); // 1=active record
??????????? header.putInt(dataLen);
??????????? header.flip();
??????????? long headerLen = header.limit();
??????????? int length = (int)(headerLen + dataLen);
??????????? long offset = getStorageLocation((int)dataLen);
??????????? //
??????????? // Store the record by key and save the offset
??????????? //
??????????? if ( offset == -1 ) {
??????????????? // We need to add to the end of the journal. Seek there
??????????????? // now only if we're not already there
??????????????? long currentPos = channel.position();
??????????????? long jounralLen = channel.size();
??????????????? if ( jounralLen != currentPos )
??????????????????? channel.position(jounralLen);
??????????????? offset = jounralLen;
??????????? }
??????????? else {
??????????????? // Seek to the returned insertion point
??????????????? channel.position(offset);
??????????? }
??????????? // Fist write the header
??????????? long written = channel.write(srcs);
??????????? // Next, see if we need to append an empty record if we inserted
??????????? // this new record at an empty location
??????????? if ( newEmptyRecordSize != -1 ) {
??????????????? // Simply write a header
??????????????? data.clear();
??????????????? data.put((byte)0);
??????????????? data.putInt(newEmptyRecordSize);
??????????????? data.flip();
??????????????? channel.write(data);
??????????? }
??????????? employeeIdx.put(emp.last, offset);
??????????? return true;
??????? }
??????? catch ( Exception e ) {
??????????? e.printStackTrace();
??????? }
??????? return false;
??? }
?
使用MappedByteBuffer的代码如下:
?
?? view plaincopy to clipboardprint?
public boolean addRecord_MBB(Employee emp) {??
??????? try {??
??????????? byte[] last = emp.last.getBytes();??
??????????? byte[] first = emp.first.getBytes();??
??????????? byte[] comments = emp.comments.getBytes();??
??????????? int datalen = last.length + first.length + comments.length + 12 + 9;??
??????????? int headerlen = 5;??
??????????? int length = headerlen + datalen;??
??????????? //??
??????????? // Store the record by key and save the offset??
??????????? //??
??????????? long offset = getStorageLocation(datalen);??
??????????? if ( offset == -1 ) {??
??????????????? // We need to add to the end of the journal. Seek there??
??????????????? // now only if we're not already there??
??????????????? long currentPos = mbb.position();??
??????????????? long journalLen = channel.size();??
??????????????? if ( (currentPos+length) >= journalLen ) {??
??????????????????? //log("GROWING FILE BY ANOTHER PAGE");??
??????????????????? mbb.force();??
??????????????????? journal.setLength(journalLen + PAGE_SIZE);??
??????????????????? channel = journal.getChannel();??
??????????????????? journalLen = channel.size();??
??????????????????? mbb = channel.map(FileChannel.MapMode.READ_WRITE, 0, journalLen);??
??????????????????? currentPos = mbb.position();??
??????????????? }??
??????????????? if ( currentEnd != currentPos )??
??????????????????? mbb.position(currentEnd);??
??????????????? offset = currentEnd;//journalLen;??
??????????? }??
??????????? else {??
??????????????? // Seek to the returned insertion point??
??????????????? mbb.position((int)offset);??
??????????? }??
??????????? // write header??
??????????? mbb.put((byte)1); // 1=active record??
??????????? mbb.putInt(datalen);??
??????????? // write data??
??????????? mbb.putInt(last.length);??
??????????? mbb.put(last);??
??????????? mbb.putInt(first.length);??
??????????? mbb.put(first);??
??????????? mbb.putInt(emp.id);??
??????????? mbb.putInt(emp.zip);??
??????????? byte employed = 0;??
??????????? if ( emp.employed )??
??????????????? employed = 1;??
??????????? mbb.put(employed);??
??????????? mbb.putInt(comments.length);??
??????????? mbb.put(comments);??
??????????? currentEnd += length;??
??????????? // Next, see if we need to append an empty record if we inserted??
??????????? // this new record at an empty location??
??????????? if ( newEmptyRecordSize != -1 ) {??
??????????????? // Simply write a header??
??????????????? mbb.put((byte)0);??
??????????????? mbb.putInt(newEmptyRecordSize);??
??????????????? currentEnd += 5;??
??????????? }??
??????????? employeeIdx.put(emp.last, offset);??
??????????? return true;??
??????? }??
??????? catch ( Exception e ) {??
??????????? e.printStackTrace();??
??????? }??
??????? return false;??
??? }?
public boolean addRecord_MBB(Employee emp) {
??????? try {
??????????? byte[] last = emp.last.getBytes();
??????????? byte[] first = emp.first.getBytes();
??????????? byte[] comments = emp.comments.getBytes();
??????????? int datalen = last.length + first.length + comments.length + 12 + 9;
??????????? int headerlen = 5;
??????????? int length = headerlen + datalen;
??????????? //
??????????? // Store the record by key and save the offset
??????????? //
??????????? long offset = getStorageLocation(datalen);
??????????? if ( offset == -1 ) {
??????????????? // We need to add to the end of the journal. Seek there
??????????????? // now only if we're not already there
??????????????? long currentPos = mbb.position();
??????????????? long journalLen = channel.size();
??????????????? if ( (currentPos+length) >= journalLen ) {
??????????????????? //log("GROWING FILE BY ANOTHER PAGE");
??????????????????? mbb.force();
??????????????????? journal.setLength(journalLen + PAGE_SIZE);
??????????????????? channel = journal.getChannel();
??????????????????? journalLen = channel.size();
??????????????????? mbb = channel.map(FileChannel.MapMode.READ_WRITE, 0, journalLen);
??????????????????? currentPos = mbb.position();
??????????????? }
??????????????? if ( currentEnd != currentPos )
??????????????????? mbb.position(currentEnd);
??????????????? offset = currentEnd;//journalLen;
??????????? }
??????????? else {
??????????????? // Seek to the returned insertion point
??????????????? mbb.position((int)offset);
??????????? }
??????????? // write header
??????????? mbb.put((byte)1); // 1=active record
??????????? mbb.putInt(datalen);
??????????? // write data
??????????? mbb.putInt(last.length);
??????????? mbb.put(last);
??????????? mbb.putInt(first.length);
??????????? mbb.put(first);
??????????? mbb.putInt(emp.id);
??????????? mbb.putInt(emp.zip);
??????????? byte employed = 0;
??????????? if ( emp.employed )
??????????????? employed = 1;
??????????? mbb.put(employed);
??????????? mbb.putInt(comments.length);
??????????? mbb.put(comments);
??????????? currentEnd += length;
??????????? // Next, see if we need to append an empty record if we inserted
??????????? // this new record at an empty location
??????????? if ( newEmptyRecordSize != -1 ) {
??????????????? // Simply write a header
??????????????? mbb.put((byte)0);
??????????????? mbb.putInt(newEmptyRecordSize);
??????????????? currentEnd += 5;
??????????? }
??????????? employeeIdx.put(emp.last, offset);
??????????? return true;
??????? }
??????? catch ( Exception e ) {
??????????? e.printStackTrace();
??????? }
??????? return false;
??? }
?接下来，调用每种方法插入100,000条记录, 耗时对比如下:
??? * With java.io: ~10,000 milliseconds
??? * With java.nio: ~2,000 milliseconds
??? * With MappedByteBuffer: ~970 milliseconds?
使用NIO的性能改善效果非常明显，使用MappedByteBuffer的性能，更是让人吃惊。使用三种方式读取数据的性能对比如下：
??? * With java.io: ~6,900 milliseconds
??? * With java.nio: ~1,400 milliseconds
??? * With MappedByteBuffer: ~355 milliseconds和写入的时候情况差不多，NIO有很明显的性能提升，而MappedByteBuffer则有惊人的高效率。从java.io迁移到nio并使用MappedByteBuffer，通常可以获得10倍以上的性能提升。