string concatenation in Java, Stringbuilder,stringbuffer and +

string concatenation in Java, Stringbuilder,stringbuffer and +, in the end which is more appropriate? StringBuilder threads are unsafe, and the efficiency is higher than thread-safe stringbuffer. Before jdk1.5, the + operation produces a large number of string objects that affect the efficiency of the GC, but after jdk1.5, the use of the + operator does not necessarily result in a large number of string objects, but is automatically optimized to StringBuilder mode. If string concatenation is performed in a For loop, the + operation is not recommended because the new StringBuilder () object is produced in large quantities.