support OS env BUFSIZE and java -DBUFSIZE to set FastWriter.BUFSIZE

solr/solrj/src/java/org/apache/solr/common/util/FastWriter.java

@@ -18,14 +18,38 @@
 
 import java.io.IOException;
 import java.io.Writer;
+import java.lang.invoke.MethodHandles;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 /** Single threaded BufferedWriter
  *  Internal Solr use only, subject to change.
  */
 public class FastWriter extends Writer {
+  private static final Logger log = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());
   // use default BUFSIZE of BufferedWriter so if we wrap that
   // it won't cause double buffering.
-  private static final int BUFSIZE = 8192;
+  private static final int BUFSIZE;
+  static {
+    int bufSize=8192;
+    final String BUF_SIZE = "BUFSIZE";
+    try {
+      String env = System.getenv(BUF_SIZE);
+      bufSize = Integer.parseInt(env);
+    } catch (Exception e) {
+      log.debug("checking OS env for "+ BUF_SIZE,e);
+      try {
+        String prop = System.getProperty(BUF_SIZE, "8192");
+        bufSize = Integer.parseInt(prop);
+      } catch (Exception ee) {
+        log.debug("checking -D property for "+ BUF_SIZE,ee);
+      }
+    } finally {
+      log.info(FastWriter.class.getSimpleName()+"."+ BUF_SIZE + "=" + bufSize);
+      BUFSIZE = bufSize;
+    }
+  }
   protected final Writer sink;
   protected char[] buf;
   protected int pos;