访问共享资源

使用互斥锁同步对从多个线程访问的变量的访问：

counter = 0
counter_mutex = Mutex.new

# Start three parallel threads and increment counter
3.times.map do |index|
  Thread.new do
    counter_mutex.synchronize { counter += 1 }
  end
end.each(&:join) # Wait for all threads to finish before killing the process

否则，当前对一个线程可见的 counter 的值可能会被另一个线程更改。

没有 Mutex 的示例 （参见例如 Thread 0，其中 Before 和 After 的差异超过 1）：

2.2.0 :224 > counter = 0; 3.times.map { |i| Thread.new { puts "[Thread #{i}] Before: #{counter}"; counter += 1; puts "[Thread #{i}] After: #{counter}"; } }.each(&:join)
[Thread 2] Before: 0
[Thread 0] Before: 0
[Thread 0] After: 2
[Thread 1] Before: 0
[Thread 1] After: 3
[Thread 2] After: 1

实施例与 Mutex：

2.2.0 :226 > mutex = Mutex.new; counter = 0; 3.times.map { |i| Thread.new { mutex.synchronize { puts "[Thread #{i}] Before: #{counter}"; counter += 1; puts "[Thread #{i}] After: #{counter}"; } } }.each(&:join)
[Thread 2] Before: 0
[Thread 2] After: 1
[Thread 1] Before: 1
[Thread 1] After: 2
[Thread 0] Before: 2
[Thread 0] After: 3