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| /**
* 继承ByteArrayOutputStream，原始 ByteArrayOutputStream 中用于接受写入 bytes 的 buf，每次空间不足时便会 new 更大容量的 byte[]，而 PoolingByteArrayOutputStream 使用了 ByteArrayPool 作为 Byte[] 缓存来减少这种操作，从而提高性能。
*
* A variation of {@link java.io.ByteArrayOutputStream} that uses a pool of byte[] buffers instead
* of always allocating them fresh, saving on heap churn.
*/
public class PoolingByteArrayOutputStream extends ByteArrayOutputStream {
/**
* 默认byte[]的大小
*
* If the {@link #PoolingByteArrayOutputStream(ByteArrayPool)} constructor is called, this is
* the default size to which the underlying byte array is initialized.
*/
private static final int DEFAULT_SIZE = 256;
/**
* byte[]的缓冲池
*/
private final ByteArrayPool mPool;
/**
* 带byte[]缓冲池的输出流，使用默认的byte[]长度
* Constructs a new PoolingByteArrayOutputStream with a default size. If more bytes are written
* to this instance, the underlying byte array will expand.
*/
public PoolingByteArrayOutputStream(ByteArrayPool pool) {
this(pool, DEFAULT_SIZE);
}
/**
* Constructs a new {@code ByteArrayOutputStream} with a default size of {@code size} bytes. If
* more than {@code size} bytes are written to this instance, the underlying byte array will
* expand.
*
* @param size initial size for the underlying byte array. The value will be pinned to a default
* minimum size.
*/
public PoolingByteArrayOutputStream(ByteArrayPool pool, int size) {
mPool = pool;
/**
* buf,该输出流将要内容写入的目标byte[],如果多次写入，buf长度不够的时候需要扩展长度
*/
buf = mPool.getBuf(Math.max(size, DEFAULT_SIZE));
}
@Override
public void close() throws IOException {
//当输出流关闭的时候，释放该byte[]回到byte缓冲池中
mPool.returnBuf(buf);
buf = null;
super.close();
}
@Override
public void finalize() {
//当垃圾回收机制准备回收该输出流时，将该byte[]回收到缓冲池
mPool.returnBuf(buf);
}
/**
* Ensures there is enough space in the buffer for the given number of additional bytes.
* 扩充当前输出流正在使用的byte[]的大小
*/
private void expand(int i) {
/* Can the buffer handle @i more bytes, if not expand it */
if (count + i <= buf.length) {
//当已经写入的字节数加上预计扩展的字节数之和，仍然不大于当前的byte[]的长度时，不需要扩展
return;
}
//当当前的byte[]不再满足时，需要从byte[]缓冲池中获取一个byte[]，大小为(count + i) * 2
byte[] newbuf = mPool.getBuf((count + i) * 2);
//将当前的byte[]内容复制到新的byte[]中
System.arraycopy(buf, 0, newbuf, 0, count);
//将旧的byte[]进行回收到byte[]缓冲池中
mPool.returnBuf(buf);
buf = newbuf;
}
/**
* 从buffer的offset位置读len长度的内容，写入到输出流的byte[]（buf）中
*/
@Override
public synchronized void write(byte[] buffer, int offset, int len) {
expand(len);
super.write(buffer, offset, len);
}
@Override
public synchronized void write(int oneByte) {
//扩展1个字节长度
expand(1);
super.write(oneByte);
}
}
|
