Will the reordering of instructions under Java single thread affect the output order of System.out.println?

Instruction reordering and System.out.println() output in Java single-threaded environment

In order to optimize program performance, the Java compiler will reorder instructions. But will this affect the output order of System.out.println() in a single-threaded program? Let's analyze it.

Consider the following code:

 System.out.println("1");
System.out.println("2");

Intuitively, you may be worried that instruction reordering causes the output order to become "2 1". However, in a single-threaded environment, the Java memory model ensures that the execution results of the program are consistent with the code sequence. Although the compiler can adjust the order of instruction execution, it must adhere to the "wappens-before" principle to ensure that the semantic correctness of a single-threaded program is not corrupted.

If reordering causes the output order of "1" and "2" to be reversed, this will mean that the execution results of the program have fundamentally changed, which violates the Java memory model's regulations. The Java memory model does not allow such complete out-of-order execution in single-threaded programs.

Therefore, in the above example, the output order of System.out.println("1"); and System.out.println("2"); does not become "21" due to instruction reordering. Only in a multi-threaded environment, due to visibility between threads, instruction reordering may cause the output to be inconsistent with expectations, but this is beyond the scope of single-thread discussion. Instruction reordering under a single thread is limited to internal optimization and does not change the final result of the program.

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