What happens when Java Compiler sees many String concatenations in one line?

It generates the equivalent of:

String s = new StringBuilder("abc")
           .append(methodReturningAString())
           .append("ghi")
           .append(anotherMethodReturningAString())
           .append("omn")
           .append("blablabla")
           .toString();

It is smart enough to pre-concatenate static strings (i.e. the "omn" + "blablabla"). You could call the use of StringBuilder a "performance trick" if you want. It is definitely better for performance than doing five concatenations resulting in four unnecessary temporary strings. Also, use of StringBuilder was a performance improvement in (I think) Java 5; prior to that, StringBuffer was used.

Edit: as pointed out in the comments, static strings are only pre-concatenated if they are at the beginning of the concatenation. Doing otherwise would break order-of-operations (although in this case I think Sun could justify it). So given this:

String s = "abc" + "def" + foo() + "uvw" + "xyz";

it would be compiled like this:

String s = new StringBuilder("abcdef")
           .append(foo())
           .append("uvw")
           .append("xyz")
           .toString();