Java IO 字節數組輸出流：ByteArrayOutputStream 源碼與實戰

ByteArrayOutputStream 定位

將字節數據寫入內存字節數組，緩衝區自動擴容（默認 32 字節，倍增策略）。最終通過 toByteArray() 或 toString() 獲取結果。常用於：內存組包、測試替換文件流、序列化緩衝。

繼承體系與核心字段

public class ByteArrayOutputStream extends OutputStream {
    protected byte buf[];  // 底層存儲數組
    protected int count;   // 有效字節數
}

關鍵源碼邏輯

擴容策略：ensureCapacity → grow

private void ensureCapacity(int minCapacity) {
    if (minCapacity - buf.length > 0) grow(minCapacity);
}
private void grow(int minCapacity) {
    int old = buf.length;
    int newCap = old << 1; // 倍增
    if (newCap - minCapacity < 0) newCap = minCapacity;
    if (newCap < 0) newCap = Integer.MAX_VALUE; // 防溢出
    buf = Arrays.copyOf(buf, newCap);
}

寫入操作

public synchronized void write(int b) {
    ensureCapacity(count + 1);
    buf[count++] = (byte)b;
}
public synchronized void write(byte[] b, int off, int len) {
    ensureCapacity(count + len);
    System.arraycopy(b, off, buf, count, len);
    count += len;
}

輸出與重置

public synchronized void writeTo(OutputStream out) throws IOException {
    out.write(buf, 0, count); // 直接輸出到另一流
}
public synchronized void reset() { count = 0; } // 重用緩衝區
public synchronized byte[] toByteArray() { return Arrays.copyOf(buf, count); }
public synchronized String toString() { return new String(buf, 0, count); }

實戰示例

ByteArrayOutputStream baos = new ByteArrayOutputStream();
baos.write(0x41); baos.write(0x42); baos.write(0x43); // ABC
baos.write(new byte[]{0x64,0x65,0x66,0x67,0x68}, 0, 5); // defgh
System.out.println(baos); // ABCdefgh
System.out.println(baos.size()); // 8

byte[] data = baos.toByteArray(); // 獲取副本
ByteArrayOutputStream baos2 = new ByteArrayOutputStream();
baos.writeTo(baos2); // 複製到另一流

總結

ByteArrayOutputStream 以動態字節數組實現內存字節累積，synchronized 保證線程安全，reset() 支持緩衝區重用減少 GC。性能敏感場景可預估大小用 new ByteArrayOutputStream(initialSize) 減少擴容開銷。