spring控制并发数的工具类ConcurrencyThrottleSupport和ConcurrencyThrottleInterceptor

官方文档：

/**
 * Support class for throttling concurrent access to a specific resource.
 *
 * <p>Designed for use as a base class, with the subclass invoking
 * the {@link #beforeAccess()} and {@link #afterAccess()} methods at
 * appropriate points of its workflow. Note that {@code afterAccess}
 * should usually be called in a finally block!
 *
 * <p>The default concurrency limit of this support class is -1
 * ("unbounded concurrency"). Subclasses may override this default;
 * check the javadoc of the concrete class that you're using.
 *
 * @author Juergen Hoeller
 * @since 1.2.5
 * @see #setConcurrencyLimit
 * @see #beforeAccess()
 * @see #afterAccess()
 * @see org.springframework.aop.interceptor.ConcurrencyThrottleInterceptor
 * @see java.io.Serializable
 */

beforeAccess()实现

/**
     * To be invoked before the main execution logic of concrete subclasses.
     * <p>This implementation applies the concurrency throttle.
     * @see #afterAccess()
     */
    protected void beforeAccess() {
        if (this.concurrencyLimit == NO_CONCURRENCY) {
            throw new IllegalStateException(
                    "Currently no invocations allowed - concurrency limit set to NO_CONCURRENCY");
        }
        if (this.concurrencyLimit > 0) {
            boolean debug = logger.isDebugEnabled();
            synchronized (this.monitor) {
                boolean interrupted = false;
                while (this.concurrencyCount >= this.concurrencyLimit) {
                    if (interrupted) {
                        throw new IllegalStateException("Thread was interrupted while waiting for invocation access, " +
                                "but concurrency limit still does not allow for entering");
                    }
                    if (debug) {
                        logger.debug("Concurrency count " + this.concurrencyCount +
                                " has reached limit " + this.concurrencyLimit + " - blocking");
                    }
                    try {
                        this.monitor.wait();
                    }
                    catch (InterruptedException ex) {
                        // Re-interrupt current thread, to allow other threads to react.
                        Thread.currentThread().interrupt();
                        interrupted = true;
                    }
                }
                if (debug) {
                    logger.debug("Entering throttle at concurrency count " + this.concurrencyCount);
                }
                this.concurrencyCount++;
            }
        }
    }

afterAccess()实现

    /**
     * To be invoked after the main execution logic of concrete subclasses.
     * @see #beforeAccess()
     */
    protected void afterAccess() {
        if (this.concurrencyLimit >= 0) {
            synchronized (this.monitor) {
                this.concurrencyCount--;
                if (logger.isDebugEnabled()) {
                    logger.debug("Returning from throttle at concurrency count " + this.concurrencyCount);
                }
                this.monitor.notify();
            }
        }
    }

ConcurrencyThrottleSupport是个抽象类，其具体的实现类ConcurrencyThrottleInterceptor

/**
 * Interceptor that throttles concurrent access, blocking invocations
 * if a specified concurrency limit is reached.
 *
 * <p>Can be applied to methods of local services that involve heavy use
 * of system resources, in a scenario where it is more efficient to
 * throttle concurrency for a specific service rather than restricting
 * the entire thread pool (e.g. the web container's thread pool).
 *
 * <p>The default concurrency limit of this interceptor is 1.
 * Specify the "concurrencyLimit" bean property to change this value.
 *
 * @author Juergen Hoeller
 * @since 11.02.2004
 * @see #setConcurrencyLimit
 */
@SuppressWarnings("serial")
public class ConcurrencyThrottleInterceptor extends ConcurrencyThrottleSupport
        implements MethodInterceptor, Serializable {

    public ConcurrencyThrottleInterceptor() {
        setConcurrencyLimit(1);
    }

    @Override
    public Object invoke(MethodInvocation methodInvocation) throws Throwable {
        beforeAccess();
        try {
            return methodInvocation.proceed();
        }
        finally {
            afterAccess();
        }
    }

}