IO流小问题

嘻嘻哈哈嘻嘻 · 嘻嘻哈哈嘻嘻

嘻嘻哈哈嘻嘻发表于 2014-9-16 16:51
这个只是从文件中读出的buf,然后写到内存中,flush()是内存中写到硬盘中

你还可以试下把读一个超过你内存的文,不去flush()看看内存会不会爆掉

kid1943 · kid1943

其实查看FileWriter类的所有方法（即包括write方法）都继承自父类OutputStreamWriter。
而查看父类OutputStream的write方法可知无论调用到哪个write方法最终还是调用到，一个叫StreamEncoder类对象se的write方法。

你找到对应的write方法，都可以发现，其会定义一个数组作为缓冲区，当缓冲区写满的时候，就会调用OutputStream（即FileWriter 在构造函数里创建的FileOutputStream对象）的write方法将数据写出。最后不难的出结论，其实FileWriter在将对应的StreamEcoder对象里的缓冲区写满的时候，数据会自动写出去即可以理解为调用了flush（这个可以看下StreamEncoder源码）。
以上是我理解的，楼主还是自己认真看一下StreamEncoder类的源码吧。

kid1943 · kid1943

kid1943 发表于 2014-10-7 09:42
其实查看FileWriter类的所有方法（即包括write方法）都继承自父类OutputStreamWriter。
而查看父类OutputSt ...

   public class StreamEncoder extends Writer
   {

      private static final int DEFAULT_BYTE_BUFFER_SIZE = 8192;

      private volatile boolean isOpen = true;

      private void ensureOpen() throws IOException {
            if (!isOpen)
               throw new IOException("Stream closed");
      }


      public static StreamEncoder forOutputStreamWriter(OutputStream out,
                                                            Object lock,
                                                         String charsetName)
            throws UnsupportedEncodingException
      {
         String csn = charsetName;
            if (csn == null)
               csn = Charset.defaultCharset().name();
            try {
               if (Charset.isSupported(csn))
                  return new StreamEncoder(out, lock, Charset.forName(csn));
            } catch (IllegalCharsetNameException x) { }
            throw new UnsupportedEncodingException (csn);
      }

      public static StreamEncoder forOutputStreamWriter(OutputStream out,
                                                         Object lock,
                                                         Charset cs)
      {
            return new StreamEncoder(out, lock, cs);
      }

      public static StreamEncoder forOutputStreamWriter(OutputStream out,
                                                         Object lock,
                                                         CharsetEncoder enc)
      {
            return new StreamEncoder(out, lock, enc);
      }



      public static StreamEncoder forEncoder(WritableByteChannel ch,
                                             CharsetEncoder enc,
                                             int minBufferCap)
      {
            return new StreamEncoder(ch, enc, minBufferCap);
      }


      public String getEncoding() {
            if (isOpen())
               return encodingName();
            return null;
      }

      public void flushBuffer() throws IOException {
         synchronized (lock) {
               if (isOpen())
                  implFlushBuffer();
               else
                  throw new IOException("Stream closed");
         }
      }

   public void write(int c) throws IOException {
         char cbuf[] = new char[1];
         cbuf[0] = (char) c;
         write(cbuf, 0, 1);
   }

   public void write(char cbuf[], int off, int len) throws IOException {
         synchronized (lock) {
               ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
               ((off + len) > cbuf.length) || ((off + len) < 0)) {
                  throw new IndexOutOfBoundsException();
            } else if (len == 0) {
               return;
            }
            implWrite(cbuf, off, len);
         }
   }

   public void write(String str, int off, int len) throws IOException {
         /* Check the len before creating a char buffer */
         if (len < 0)
            throw new IndexOutOfBoundsException();
         char cbuf[] = new char[len];
      str.getChars(off, off + len, cbuf, 0);
         write(cbuf, 0, len);
      }

      public void flush() throws IOException {
         synchronized (lock) {
            ensureOpen();
            implFlush();
         }
      }

      public void close() throws IOException {
         synchronized (lock) {
               if (!isOpen)
                  return;
               implClose();
            isOpen = false;
         }
      }

      private boolean isOpen() {
         return isOpen;
      }



      private Charset cs;
      private CharsetEncoder encoder;
      private ByteBuffer bb;


      private final OutputStream out;
      private WritableByteChannel ch;

      private boolean haveLeftoverChar = false;
      private char leftoverChar;
      private CharBuffer lcb = null;

      private StreamEncoder(OutputStream out, Object lock, Charset cs) {
         this(out, lock,
            cs.newEncoder()
            .onMalformedInput(CodingErrorAction.REPLACE)
         .onUnmappableCharacter(CodingErrorAction.REPLACE));
      }

      private StreamEncoder(OutputStream out, Object lock, CharsetEncoder enc) {


         super(lock);
         this.out = out;
         this.ch = null;
         this.cs = enc.charset();
         this.encoder = enc;

         // This path disabled until direct buffers are faster
      if (false && out instanceof FileOutputStream) {
               ch = ((FileOutputStream)out).getChannel();
         if (ch != null)
                     bb = ByteBuffer.allocateDirect(DEFAULT_BYTE_BUFFER_SIZE);
         }
            if (ch == null) {
         bb = ByteBuffer.allocate(DEFAULT_BYTE_BUFFER_SIZE);
         }
      }

      private StreamEncoder(WritableByteChannel ch, CharsetEncoder enc, int mbc) {
         this.out = null;
         this.ch = ch;
         this.cs = enc.charset();
         this.encoder = enc;
         this.bb = ByteBuffer.allocate(mbc < 0
                                 ? DEFAULT_BYTE_BUFFER_SIZE
                                 : mbc);
   }

      private void writeBytes() throws IOException {
         bb.flip();

         int lim = bb.limit();
         int pos = bb.position();
         assert (pos <= lim);
         int rem = (pos <= lim ? lim - pos : 0);

            if (rem > 0) {
         if (ch != null) {
               if (ch.write(bb) != rem)
                  assert false : rem;
         } else {
            out.write(bb.array(), bb.arrayOffset() + pos, rem);
         }
         }
         bb.clear();
      }

      private void flushLeftoverChar(CharBuffer cb, boolean endOfInput)
         throws IOException
      {
         if (!haveLeftoverChar && !endOfInput)
            return;
         if (lcb == null)
            lcb = CharBuffer.allocate(2);
         else
            lcb.clear();
         if (haveLeftoverChar)
            lcb.put(leftoverChar);
      if ((cb != null) && cb.hasRemaining())
            lcb.put(cb.get());
      lcb.flip();
         while (lcb.hasRemaining() || endOfInput) {
            CoderResult cr = encoder.encode(lcb, bb, endOfInput);
            if (cr.isUnderflow()) {
               if (lcb.hasRemaining()) {
                     leftoverChar = lcb.get();
                  if (cb != null && cb.hasRemaining())
                        flushLeftoverChar(cb, endOfInput);
                  return;
               }
               break;
            }
         if (cr.isOverflow()) {
                  assert bb.position() > 0;
                  writeBytes();
                  continue;
            }
            cr.throwException();
      }
         haveLeftoverChar = false;
   }

         void implWrite(char cbuf[], int off, int len)
            throws IOException
         {
            CharBuffer cb = CharBuffer.wrap(cbuf, off, len);

            if (haveLeftoverChar)
            flushLeftoverChar(cb, false);
               while (cb.hasRemaining()) {
            CoderResult cr = encoder.encode(cb, bb, false);/
            if (cr.isUnderflow()) {

               assert (cb.remaining() <= 1) : cb.remaining();、
               if (cb.remaining() == 1) {
                     haveLeftoverChar = true;
                     leftoverChar = cb.get();
               }
               break;
            }
            if (cr.isOverflow()) {
               assert bb.position() > 0;
               writeBytes();
               continue;
            }
            cr.throwException();
         }


      void implFlushBuffer() throws IOException {
         if (bb.position() > 0)
         writeBytes();
      }

      void implFlush() throws IOException {
         implFlushBuffer();
         if (out != null)
         out.flush();}

      void implClose() throws IOException {
         flushLeftoverChar(null, true);
         try {
               for (;;) {
                  CoderResult cr = encoder.flush(bb);
                  if (cr.isUnderflow())
                     break;
                  if (cr.isOverflow()) {
                     assert bb.position() > 0;
                     writeBytes();
                     continue;
                  }
                  cr.throwException();
               }

               if (bb.position() > 0)
                  writeBytes();
               if (ch != null)
                  ch.close();
            else
                  out.close();
         } catch (IOException x) {
            encoder.reset();
            throw x;
         }
      }

      String encodingName() {
         return ((cs instanceof HistoricallyNamedCharset)
            ? ((HistoricallyNamedCharset)cs).historicalName()
            : cs.name());
      }
   }

kid1943 · kid1943

kid1943 发表于 2014-10-7 09:42
其实查看FileWriter类的所有方法（即包括write方法）都继承自父类OutputStreamWriter。
而查看父类OutputSt ...

public class StreamEncoder extends Writer
{
private static final int DEFAULT_BYTE_BUFFER_SIZE = 8192;
private volatile boolean isOpen = true;
private void ensureOpen() throws IOException {
if (!isOpen)
throw new IOException("Stream closed");
}
public static StreamEncoder forOutputStreamWriter(OutputStream out,
Object lock,
String charsetName)
throws UnsupportedEncodingException
{
String csn = charsetName;
if (csn == null)
csn = Charset.defaultCharset().name();
try {
if (Charset.isSupported(csn))
return new StreamEncoder(out, lock, Charset.forName(csn));
} catch (IllegalCharsetNameException x) { }
throw new UnsupportedEncodingException (csn);
}
public static StreamEncoder forOutputStreamWriter(OutputStream out,
Object lock,
Charset cs)
{
return new StreamEncoder(out, lock, cs);
}
public static StreamEncoder forOutputStreamWriter(OutputStream out,
Object lock,
CharsetEncoder enc)
{
return new StreamEncoder(out, lock, enc);
}
public static StreamEncoder forEncoder(WritableByteChannel ch,
CharsetEncoder enc,
int minBufferCap)
{
return new StreamEncoder(ch, enc, minBufferCap);
}
public String getEncoding() {
if (isOpen())
return encodingName();
return null;
}
public void flushBuffer() throws IOException {
synchronized (lock) {
if (isOpen())
implFlushBuffer();
else
throw new IOException("Stream closed");
}
}
public void write(int c) throws IOException {
char cbuf[] = new char[1];
cbuf[0] = (char) c;
write(cbuf, 0, 1);
}
public void write(char cbuf[], int off, int len) throws IOException {
synchronized (lock) {
ensureOpen();
if ((off < 0) || (off > cbuf.length) || (len < 0) ||
((off + len) > cbuf.length) || ((off + len) < 0)) {
throw new IndexOutOfBoundsException();
} else if (len == 0) {
return;
}
implWrite(cbuf, off, len);
}
}
public void write(String str, int off, int len) throws IOException {
/* Check the len before creating a char buffer */
if (len < 0)
throw new IndexOutOfBoundsException();
char cbuf[] = new char[len];
str.getChars(off, off + len, cbuf, 0);
write(cbuf, 0, len);
}
public void flush() throws IOException {
synchronized (lock) {
ensureOpen();
implFlush();
}
}
public void close() throws IOException {
synchronized (lock) {
if (!isOpen)
return;
implClose();
isOpen = false;
}
}
private boolean isOpen() {
return isOpen;
}
private Charset cs;
private CharsetEncoder encoder;
private ByteBuffer bb;
private final OutputStream out;
private WritableByteChannel ch;
private boolean haveLeftoverChar = false;
private char leftoverChar;
private CharBuffer lcb = null;
private StreamEncoder(OutputStream out, Object lock, Charset cs) {
this(out, lock,
cs.newEncoder()
.onMalformedInput(CodingErrorAction.REPLACE)
.onUnmappableCharacter(CodingErrorAction.REPLACE));
}
private StreamEncoder(OutputStream out, Object lock, CharsetEncoder enc) {
super(lock);
this.out = out;
this.ch = null;
this.cs = enc.charset();
this.encoder = enc;
// This path disabled until direct buffers are faster
if (false && out instanceof FileOutputStream) {
ch = ((FileOutputStream)out).getChannel();
if (ch != null)
bb = ByteBuffer.allocateDirect(DEFAULT_BYTE_BUFFER_SIZE);
}
if (ch == null) {
bb = ByteBuffer.allocate(DEFAULT_BYTE_BUFFER_SIZE);
}
}
private StreamEncoder(WritableByteChannel ch, CharsetEncoder enc, int mbc) {
this.out = null;
this.ch = ch;
this.cs = enc.charset();
this.encoder = enc;
this.bb = ByteBuffer.allocate(mbc < 0
? DEFAULT_BYTE_BUFFER_SIZE
: mbc);
}
private void writeBytes() throws IOException {
bb.flip();
int lim = bb.limit();
int pos = bb.position();
assert (pos <= lim);
int rem = (pos <= lim ? lim - pos : 0);
if (rem > 0) {
if (ch != null) {
if (ch.write(bb) != rem)
assert false : rem;
} else {
out.write(bb.array(), bb.arrayOffset() + pos, rem);
}
}
bb.clear();
}
private void flushLeftoverChar(CharBuffer cb, boolean endOfInput)
throws IOException
{
if (!haveLeftoverChar && !endOfInput)
return;
if (lcb == null)
lcb = CharBuffer.allocate(2);
else
lcb.clear();
if (haveLeftoverChar)
lcb.put(leftoverChar);
if ((cb != null) && cb.hasRemaining())
lcb.put(cb.get());
lcb.flip();
while (lcb.hasRemaining() || endOfInput) {
CoderResult cr = encoder.encode(lcb, bb, endOfInput);
if (cr.isUnderflow()) {
if (lcb.hasRemaining()) {
leftoverChar = lcb.get();
if (cb != null && cb.hasRemaining())
flushLeftoverChar(cb, endOfInput);
return;
}
break;
}
if (cr.isOverflow()) {
assert bb.position() > 0;
writeBytes();
continue;
}
cr.throwException();
}
haveLeftoverChar = false;
}
void implWrite(char cbuf[], int off, int len)
throws IOException
{
CharBuffer cb = CharBuffer.wrap(cbuf, off, len);
if (haveLeftoverChar)
flushLeftoverChar(cb, false);
while (cb.hasRemaining()) {
CoderResult cr = encoder.encode(cb, bb, false);/
if (cr.isUnderflow()) {
assert (cb.remaining() <= 1) : cb.remaining();、
if (cb.remaining() == 1) {
haveLeftoverChar = true;
leftoverChar = cb.get();
}
break;
}
if (cr.isOverflow()) {
assert bb.position() > 0;
writeBytes();
continue;
}
cr.throwException();
}
void implFlushBuffer() throws IOException {
if (bb.position() > 0)
writeBytes();
}
void implFlush() throws IOException {
implFlushBuffer();
if (out != null)
out.flush();}
void implClose() throws IOException {
flushLeftoverChar(null, true);
try {
for (;;) {
CoderResult cr = encoder.flush(bb);
if (cr.isUnderflow())
break;
if (cr.isOverflow()) {
assert bb.position() > 0;
writeBytes();
continue;
}
cr.throwException();
}
if (bb.position() > 0)
writeBytes();
if (ch != null)
ch.close();
else
out.close();
} catch (IOException x) {
encoder.reset();
throw x;
}
}
String encodingName() {
return ((cs instanceof HistoricallyNamedCharset)
? ((HistoricallyNamedCharset)cs).historicalName()
: cs.name());
}
}
这是StreamEncoder源码

复制代码

郑飞 · 郑飞

kid1943 发表于 2014-10-7 09:42
其实查看FileWriter类的所有方法（即包括write方法）都继承自父类OutputStreamWriter。
而查看父类OutputSt ...

后来是看了源码确实是封装了flush对象谢谢...

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