递归调用的思考

递归，就是自己调用自己，比如：

int foo(int n) {
	if (n == 1) {
		return 1;
	}
	return foo(n - 1) + 1;
}

• 每调用一次方法， 就会在当前线程的虚拟机栈中分配一块内存空间，称为栈帧。
• 递归调用时，次数越多，入栈的栈帧越多，最终导致栈溢出（StackOverflowError）。

	static int deep;

	public void stackTest() {
		deep++;
		stackTest();
	}

	public static void main(String[] args) {
		StackOverflowTest test = new StackOverflowTest();

		try {
			test.stackTest();
		} catch (Throwable e) {
			System.out.println(e.toString());
			System.out.println(deep);
		}
	}

递归一定深度后直接返回结果，由外部变量保存，接着继续递归

public class Test {

	public static void main(String[] args) {
		int result = 0;

		Test test = new Test();
		do {
			result = test.foo(result, 0);
		} while (!test.isEnd(result));

		System.out.println("程序运行结束 --- result = " + result);
	}

	private int maxDeep = 1000;

	private int foo(int count, int deep) {
		if (isEnd(count)) {
			return count;
		}

		if (deep >= maxDeep) {
			System.out.println(Thread.currentThread().getName() + " --- " + count);
			return count;
		}

		count++;
		deep++;

		return foo(count, deep);
	}

	public boolean isEnd(int count) {
		return count >= 1000000;
	}

}

因为虚拟机栈是线程私有的，递归一定次数后，后续的递归调用让新线程来处理，减轻栈内存压力

public class Test {

	public static void main(String[] args) throws InterruptedException, ExecutionException {
		int result = 0;

		// 线程池复用
		ExecutorService executorService = Executors.newSingleThreadExecutor();
		do {
			result = executorService.submit(new MyTask(result)).get();
		} while (!MyTask.isEnd(result));

		executorService.shutdown();
		System.out.println("程序运行结束 --- result = " + result);

	}

}

public class MyTask implements Callable {

	public MyTask(int count) {
		this.count = count;
	}

	private int count;

	private int maxDeep = 1000;
	private int deep = 0;

	@Override
	public Integer call() throws Exception {
		return foo(count);
	}

	private int foo(int count) {
		if (isEnd(count)) {
			return count;
		}

		if (deep >= maxDeep) {
			System.out.println(Thread.currentThread().getName() + " --- " + count);
			return count;
		}

		count++;
		deep++;

		return foo(count);
	}

	public static boolean isEnd(int count) {
		return count >= 1000000;
	}

}