Summary

Add a new constructor to the GZIPOutputStream that allows the caller to provide a custom Deflater instance.

Problem

The class GZIPOutputStream extends DeflaterOutputStream, which is logical because the GZIP encoding is based on ZLIB "deflate" encoding.

However, while DeflaterOutputStream provides constructors that take a custom Deflater argument supplied by the caller, GZIPOutputStream has no such constructors.

As a result, it's not possible to do entirely reasonable customization, such as configuring a GZIPOutputStream for a non-default compression level.

To quote Mark Reinhold, asking for such a constructor "is not an unreasonable request" (May 14, 2001).

Solution

Add a new constructor taking all of the existing parameters (size and syncFlush) and adding a Deflater parameter.

Another option would be to add multiple constructors, with all combinations with/without size and syncFlush which is what DeflaterOutputStream does.

However, that was deemed overkill. A single new constructor which takes all of the other optional parameters simply requires those users who need to supply a custom Deflater to also supply those parameters, which is not hard, especially if you are user who's sophisticated enough to know what a Deflater is.

Specification

diff --git a/src/java.base/share/classes/java/util/zip/GZIPOutputStream.java b/src/java.base/share/classes/java/util/zip/GZIPOutputStream.java
index cea0880fca6..8d6ce5d8452 100644
--- a/src/java.base/share/classes/java/util/zip/GZIPOutputStream.java
+++ b/src/java.base/share/classes/java/util/zip/GZIPOutputStream.java
@@ -59,6 +59,38 @@ public class GZIPOutputStream extends DeflaterOutputStream {
     // Represents the default "unknown" value for OS header, per RFC-1952
     private static final byte OS_UNKNOWN = (byte) 255;

+    /**
+     * Creates a new output stream with the specified compressor,
+     * buffer size and flush mode.
+     *
+     * <p>
+     * The given {@link Deflater} will not be closed by this instance. To release
+     * the resources used by {@code def}, an application must explicitly close it
+     * by invoking its {@link Deflater#end end()} or {@link Deflater#close close()}
+     * method.
+     *
+     * @param out the output stream
+     * @param def the compressor ("deflater")
+     * @param size the output buffer size
+     * @param syncFlush
+     *        if {@code true} invocation of the inherited
+     *        {@link DeflaterOutputStream#flush() flush()} method of
+     *        this instance flushes the compressor with flush mode
+     *        {@link Deflater#SYNC_FLUSH} before flushing the output
+     *        stream, otherwise only flushes the output stream
+     * @throws    IOException If an I/O error has occurred.
+     * @throws    IllegalArgumentException if {@code size <= 0}
+     *
+     * @since 25
+     */
+    public GZIPOutputStream(OutputStream out, Deflater def, int size, boolean syncFlush)
+        throws IOException
+    {
+        super(out, def, size, syncFlush);
+        writeHeader();
+        crc.reset();
+    }
+
     /**
      * Creates a new output stream with the specified buffer size.
      *