我试图向学生说明 Java 中传统 IO 和内存映射文件之间的性能差异。我在互联网上的某个地方找到了一个例子，但对我来说并不是一切都清楚，我什至不认为所有步骤都是必要的。我在这里和那里阅读了很多关于它的内容，但我不相信它们都不能正确实现。
我试图理解的代码是：
public class FileCopy{
public static void main(String args[]){
if (args.length < 1){
System.out.println(" Wrong usage!");
System.out.println(" Correct usage is : java FileCopy <large file with full path>");
System.exit(0);
}
String inFileName = args[0];
File inFile = new File(inFileName);
if (inFile.exists() != true){
System.out.println(inFileName + " does not exist!");
System.exit(0);
}
try{
new FileCopy().memoryMappedCopy(inFileName, inFileName+".new" );
new FileCopy().customBufferedCopy(inFileName, inFileName+".new1");
}catch(FileNotFoundException fne){
fne.printStackTrace();
}catch(IOException ioe){
ioe.printStackTrace();
}catch (Exception e){
e.printStackTrace();
}
}
public void memoryMappedCopy(String fromFile, String toFile ) throws Exception{
long timeIn = new Date().getTime();
// read input file
RandomAccessFile rafIn = new RandomAccessFile(fromFile, "rw");
FileChannel fcIn = rafIn.getChannel();
ByteBuffer byteBuffIn = fcIn.map(FileChannel.MapMode.READ_WRITE, 0,(int) fcIn.size());
fcIn.read(byteBuffIn);
byteBuffIn.flip();
RandomAccessFile rafOut = new RandomAccessFile(toFile, "rw");
FileChannel fcOut = rafOut.getChannel();
ByteBuffer writeMap = fcOut.map(FileChannel.MapMode.READ_WRITE,0,(int) fcIn.size());
writeMap.put(byteBuffIn);
long timeOut = new Date().getTime();
System.out.println("Memory mapped copy Time for a file of size :" + (int) fcIn.size() +" is "+(timeOut-timeIn));
fcOut.close();
fcIn.close();
}
static final int CHUNK_SIZE = 100000;
static final char[] inChars = new char[CHUNK_SIZE];
public static void customBufferedCopy(String fromFile, String toFile) throws IOException{
long timeIn = new Date().getTime();
Reader in = new FileReader(fromFile);
Writer out = new FileWriter(toFile);
while (true) {
synchronized (inChars) {
int amountRead = in.read(inChars);
if (amountRead == -1) {
break;
}
out.write(inChars, 0, amountRead);
}
}
long timeOut = new Date().getTime();
System.out.println("Custom buffered copy Time for a file of size :" + (int) new File(fromFile).length() +" is "+(timeOut-timeIn));
in.close();
out.close();
}
}
究竟什么时候需要使用RandomAccessFile？在这里它是用来读写的memoryMappedCopy，只是复制一个文件真的有必要吗？还是它是内存映射的一部分？
在customBufferedCopy，为什么synchronized在这里使用？
我还发现了一个不同的例子——应该——测试两者之间的性能：
public class MappedIO {
private static int numOfInts = 4000000;
private static int numOfUbuffInts = 200000;
private abstract static class Tester {
private String name;
public Tester(String name) { this.name = name; }
public long runTest() {
System.out.print(name + ": ");
try {
long startTime = System.currentTimeMillis();
test();
long endTime = System.currentTimeMillis();
return (endTime - startTime);
} 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.tmp"))));
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.tmp", "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 Read") {
public void test() throws IOException {
DataInputStream dis = new DataInputStream(
new BufferedInputStream(
new FileInputStream("temp.tmp")));
for(int i = 0; i < numOfInts; i++)
dis.readInt();
dis.close();
}
},
new Tester("Mapped Read") {
public void test() throws IOException {
FileChannel fc = new FileInputStream(
new File("temp.tmp")).getChannel();
IntBuffer ib = fc.map(
FileChannel.MapMode.READ_ONLY, 0, fc.size())
.asIntBuffer();
while(ib.hasRemaining())
ib.get();
fc.close();
}
},
new Tester("Stream Read/Write") {
public void test() throws IOException {
RandomAccessFile raf = new RandomAccessFile(
new File("temp.tmp"), "rw");
raf.writeInt(1);
for(int i = 0; i < numOfUbuffInts; i++) {
raf.seek(raf.length() - 4);
raf.writeInt(raf.readInt());
}
raf.close();
}
},
new Tester("Mapped Read/Write") {
public void test() throws IOException {
FileChannel fc = new RandomAccessFile(
new File("temp.tmp"), "rw").getChannel();
IntBuffer ib = fc.map(
FileChannel.MapMode.READ_WRITE, 0, fc.size())
.asIntBuffer();
ib.put(0);
for(int i = 1; i < numOfUbuffInts; i++)
ib.put(ib.get(i - 1));
fc.close();
}
}
};
public static void main(String[] args) {
for(int i = 0; i < tests.length; i++)
System.out.println(tests[i].runTest());
}
}
我或多或少看到发生了什么，我的输出如下所示：
Stream Write: 653
Mapped Write: 51
Stream Read: 651
Mapped Read: 40
Stream Read/Write: 14481
Mapped Read/Write: 6
是什么让 Stream Read/Write 如此长的令人难以置信？作为一个读/写测试，对我来说，一遍又一遍地读取相同的整数看起来有点毫无意义（如果我很好地理解 中发生了什么Stream Read/Write）从先前写入的文件中读取 int 不是更好吗？在同一个地方读写整数？有没有更好的方法来说明它？
一段时间以来，我一直对很多这些事情感到头疼，但我无法了解整个情况。