StringBuilder is not thread safe. So, it performs better in situations where thread safety is not required.

StringBuffer is implemented by using synchronized keyword on all methods.

 

StringBuffer

/*      */   public synchronized StringBuffer append(String string)/*      */   {/*  197 */     if (string == null) string = String.valueOf(string);/*  198 */     int adding = string.length();/*  199 */     int newSize = this.count + adding;/*  200 */     if (newSize > this.value.length) {/*  201 */       ensureCapacityImpl(newSize);/*      */     }/*  203 */     string.getChars(0, adding, this.value, this.count);/*  204 */     this.count = newSize;/*  205 */     return this;/*      */   }

StringBuilder

/*      */   public StringBuilder append(String string)/*      */   {/*  201 */     if (string == null) string = String.valueOf(string);/*  202 */     int adding = string.length();/*  203 */     int newSize = this.count + adding;/*  204 */     if (newSize > this.value.length) {/*  205 */       ensureCapacityImpl(newSize);/*      */     }/*  207 */     string.getChars(0, adding, this.value, this.count);/*  208 */     this.count = newSize;/*  209 */     return this;/*      */   }