RFR [9]8134424: BlockDataInputStream.readUTFBody: examine sizing local StringBuffer with the given length

[Chris Hegarty]

It was suggested to me off-list that the implementation should choose a
reasonable initial capacity value ,to size the StringBuilder, rather than
the value read from the stream ( in case of bad or corrupt data ). So the 
proposed changes are:

diff --git a/src/java.base/share/classes/java/io/ObjectInputStream.java b/src/java.base/share/classes/java/io/ObjectInputStream.java
--- a/src/java.base/share/classes/java/io/ObjectInputStream.java
+++ b/src/java.base/share/classes/java/io/ObjectInputStream.java
@@ -3144,7 +3144,9 @@
          * utflen bytes.
          */
         private String readUTFBody(long utflen) throws IOException {
-            StringBuilder sbuf = new StringBuilder();
+            // a reasonably initial capacity based on the UTF length
+            int initialCapacity = Math.max((int)utflen, 10_000);
+            StringBuilder sbuf = new StringBuilder(initialCapacity);
             if (!blkmode) {
                 end = pos = 0;
             }



-Chris.

On 8 Feb 2016, at 11:15, Chris Hegarty <chris.hegarty at oracle.com> wrote:

> Low hanging fruit to avoid unnecessary allocations when deserializing.
> readUTF knows the string size ahead of the read and can avoid
> expandCapacity() by constructing the StringBuilder with the expected size. 
> 
> It is an implementation detail, but if the size is larger than Integer.MAX_VALUE,
> then OOM can be thrown, as is the case in the implementation today.
> 
> diff --git a/src/java.base/share/classes/java/io/ObjectInputStream.java b/src/java.base/share/classes/java/io/ObjectInputStream.java
> --- a/src/java.base/share/classes/java/io/ObjectInputStream.java
> +++ b/src/java.base/share/classes/java/io/ObjectInputStream.java
> @@ -3144,7 +3144,9 @@
>          * utflen bytes.
>          */
>         private String readUTFBody(long utflen) throws IOException {
> -            StringBuilder sbuf = new StringBuilder();
> +            if (utflen > Integer.MAX_VALUE)
> +                throw new OutOfMemoryError("UTF length, " + utflen + ", too big.");
> +            StringBuilder sbuf = new StringBuilder((int)utflen);
>             if (!blkmode) {
>                 end = pos = 0;
>             }
> 
> -Chris.