基于Thread的多wait触发场景下，主线程notify处理方案咨询

Hey David, let's work through the problems you're hitting with multiple wait triggers in your fillBlocks method. First, let's spot the critical issues in your current code and then fix them with proper wait/notify best practices.

1. Critical Mistake: Calling wait() Outside a Synchronized Block #

Your current code calls ads.wait() without wrapping it in a synchronized(ads) block. This will immediately throw an IllegalMonitorStateException at runtime—because a thread must hold the object's monitor lock to call wait(), notify(), or notifyAll().

2. Fixing Spurious Wakeups & Multiple Wait Triggers #

The biggest problem with handling multiple wait events is spurious wakeups (threads waking up without a corresponding notify()) and ensuring only threads that meet the condition proceed after being awakened. Using an if check isn't enough here—you need a while loop to re-verify the condition every time the thread wakes up.

Here's the corrected version of your fillBlocks method:

private void fillBlocks(){
    ASCIIDataStream ads = new ASCIIDataStream(this.getRawCode());
    ASCIIDataStream aux = new ASCIIDataStream("");
    DataStream<?, ?>[] bds = null;

    synchronized(ads) { // Must hold the monitor to call wait/notify
        while ((ads.getProcessedChars() < this.BLOCK_LENGTH) && (!ads.isDone())) {
            try {
                ads.wait(); // Wait until condition is met or interrupted
            } catch (InterruptedException e) {
                e.printStackTrace();
                // Restore interrupt status for upper-level handling
                Thread.currentThread().interrupt();
                // Exit method if interruption should halt processing
                return;
            }
        }
    }

    // Proceed with block-filling logic once condition is satisfied
    // ... Your processing code here ...
}

Key Explanations #

• Synchronized Block: Wrapping the wait logic in synchronized(ads) ensures your thread holds the monitor lock required for safe wait() calls.
• While Loop Instead of If: Every time the thread wakes up (whether from notify(), notifyAll(), or a spurious wakeup), it rechecks the condition. If the condition still isn't met, it goes back to waiting—this is essential for handling multiple wait triggers correctly.
• Interruption Handling: Restoring the thread's interrupt status lets upper-level code respond to the interruption properly, instead of silently ignoring it.

Additional Best Practices for Multiple Wait Scenarios #

• Use notifyAll() Instead of notify(): When updating ads state (e.g., when processedChars reaches BLOCK_LENGTH or isDone becomes true), call ads.notifyAll(). This ensures all waiting threads get a chance to recheck the condition, preventing threads from getting stuck indefinitely.
• Align Notify Triggers with Wait Conditions: Always call notifyAll() immediately after modifying the state that your threads are waiting on. For example, if another thread adds data to ads and increases processedChars, trigger a notify right after the update.
• Minimize Locked Code: Keep the code inside the synchronized block as minimal as possible—only the condition check and wait() call. This reduces lock contention between threads.

内容的提问来源于stack exchange，提问作者David