我目前正在开发一个需要随机访问许多（60k-100k）相对大文件的应用程序。由于打开和关闭流是一项相当昂贵的操作，因此我更愿意将最大文件的 FileChannels 保持打开，直到不再需要它们为止。
问题是，由于 Java 7 的 try-with 语句没有涵盖这种行为，因此我需要手动关闭所有 FileChannel。但这变得越来越复杂，因为可以在整个软件中同时访问相同的文件。
我已经实现了一个 ChannelPool 类，它可以跟踪每个注册路径的打开的 FileChannel 实例。然后可以发出 ChannelPool 以关闭那些 Path 仅在特定时间间隔内被池本身弱引用的通道。我更喜欢事件侦听器方法，但我也不想听 GC。
来自 Apache Commons的FileChannelPool没有解决我的问题，因为通道仍然需要手动关闭。
这个问题有更优雅的解决方案吗？如果没有，如何改进我的实施？
import java.io.IOException;
import java.lang.ref.WeakReference;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.Map;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.ConcurrentHashMap;
public class ChannelPool {
private static final ChannelPool defaultInstance = new ChannelPool();
private final ConcurrentHashMap<String, ChannelRef> channels;
private final Timer timer;
private ChannelPool(){
channels = new ConcurrentHashMap<>();
timer = new Timer();
}
public static ChannelPool getDefault(){
return defaultInstance;
}
public void initCleanUp(){
// wait 2 seconds then repeat clean-up every 10 seconds.
timer.schedule(new CleanUpTask(this), 2000, 10000);
}
public void shutDown(){
// must be called manually.
timer.cancel();
closeAll();
}
public FileChannel getChannel(Path path){
ChannelRef cref = channels.get(path.toString());
System.out.println("getChannel called " + channels.size());
if (cref == null){
cref = ChannelRef.newInstance(path);
if (cref == null){
// failed to open channel
return null;
}
ChannelRef oldRef = channels.putIfAbsent(path.toString(), cref);
if (oldRef != null){
try{
// close new channel and let GC dispose of it
cref.channel().close();
System.out.println("redundant channel closed");
}
catch (IOException ex) {}
cref = oldRef;
}
}
return cref.channel();
}
private void remove(String str) {
ChannelRef ref = channels.remove(str);
if (ref != null){
try {
ref.channel().close();
System.out.println("old channel closed");
}
catch (IOException ex) {}
}
}
private void closeAll() {
for (Map.Entry<String, ChannelRef> e : channels.entrySet()){
remove(e.getKey());
}
}
private void maintain() {
// close channels for derefenced paths
for (Map.Entry<String, ChannelRef> e : channels.entrySet()){
ChannelRef ref = e.getValue();
if (ref != null){
Path p = ref.pathRef().get();
if (p == null){
// gc'd
remove(e.getKey());
}
}
}
}
private static class ChannelRef{
private FileChannel channel;
private WeakReference<Path> ref;
private ChannelRef(FileChannel channel, WeakReference<Path> ref) {
this.channel = channel;
this.ref = ref;
}
private static ChannelRef newInstance(Path path) {
FileChannel fc;
try {
fc = FileChannel.open(path, StandardOpenOption.READ);
}
catch (IOException ex) {
return null;
}
return new ChannelRef(fc, new WeakReference<>(path));
}
private FileChannel channel() {
return channel;
}
private WeakReference<Path> pathRef() {
return ref;
}
}
private static class CleanUpTask extends TimerTask {
private ChannelPool pool;
private CleanUpTask(ChannelPool pool){
super();
this.pool = pool;
}
@Override
public void run() {
pool.maintain();
pool.printState();
}
}
private void printState(){
System.out.println("Clean up performed. " + channels.size() + " channels remain. -- " + System.currentTimeMillis());
for (Map.Entry<String, ChannelRef> e : channels.entrySet()){
ChannelRef cref = e.getValue();
String out = "open: " + cref.channel().isOpen() + " - " + cref.channel().toString();
System.out.println(out);
}
}
}
编辑：感谢fge的回答，我现在完全有我需要的东西。谢谢！
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.cache.RemovalListener;
import com.google.common.cache.RemovalNotification;
import java.io.IOException;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;
import java.util.concurrent.ExecutionException;
public class Channels {
private static final LoadingCache<Path, FileChannel> channelCache =
CacheBuilder.newBuilder()
.weakKeys()
.removalListener(
new RemovalListener<Path, FileChannel>(){
@Override
public void onRemoval(RemovalNotification<Path, FileChannel> removal) {
FileChannel fc = removal.getValue();
try {
fc.close();
}
catch (IOException ex) {}
}
}
)
.build(
new CacheLoader<Path, FileChannel>() {
@Override
public FileChannel load(Path path) throws IOException {
return FileChannel.open(path, StandardOpenOption.READ);
}
}
);
public static FileChannel get(Path path){
try {
return channelCache.get(path);
}
catch (ExecutionException ex){}
return null;
}
}