I don't know if that's the answer you're looking for, but a call to append is synchronized: public synchronized StringBuffer append(/* Something */); ...

I don't know if that's the answer you're looking for, but a call to append is synchronized: public synchronized StringBuffer append(/* Something */); ... but chaining them doesn't mean that the chain is synchronized. sb.append(key).append("=").append(value).append(", "); is executed the same way as sb.append(key); sb.append("="); sb.append(value); sb.append(", "); Two concurrent threads running this part of code might do this: sb.append(key); // # 1 sb.append("="); // # 2 sb.append(value); // # 3 sb.append(", "); // # 4 // Thread 1 executes #1 // Thread 2 executes #1 // Thread 1 executes #2 // Thread 1 executes #3 // Thread 2 executes #2 // Thread 2 executes #3 // Thread 2 executes #4 // Thread 1 executes #4 This can be executed in many more different ways. To ensure that only one thread executes those `append` at once, you would have to synchronize on the `StringBuffer` instance: synchronize(sb){ sb.append(key).append("=").append(value).append(", "); } Then only one thread will do the appending at a time. But this introduce yet another lock in your code, and render the `synchronized` methods useless and redondant.