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。
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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：
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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：
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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：
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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：
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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:
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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。
源文档 <http://blog.csdn.net/sean1203/archive/2010/01/06/5142464.aspx>