類中字段賦值給局部變量後再使用意義何在？

public class ArrayBlockingQueue {
	private final ReentrantLock lock;
	
	//...other fields and methods
	
	public int remainingCapacity() {
		final ReentrantLock lock = this.lock;
		lock.lock();
		try {
			return items.length - count;
		} finally {
			lock.unlock();
		}
	}
}

public class ArrayBlockingQueue {
	private final ReentrantLock lock;
	
	//...other fields and methods
	
	public int remainingCapacity() {
		this.lock.lock();
		try {
			return items.length - count;
		} finally {
			this.lock.unlock();
		}
	}
}

歸根究底是由於內存模型與OOP之間的原則不一致。

幾乎j.u.c包中的每個方法都采用了這樣一種策略：當一個值會被多次使用時，就將這個字段讀出來賦值給局部變量。雖然這種做法不雅觀，但檢查起來會更直觀。

final字段也會做這樣處理，可能有些令人不解。這是因為JVM並不足夠智能，不能充分利用JMM已經提供了安全保證的可優化點，比如可以不用重新加載final值到緩存。相比過去，JVM在這方麵有很大進步，但仍不夠智能。

It’s ultimately due to the fundamental mismatch between memory models
and OOP

Just about every method in all of j.u.c adopts the policy of reading fields as locals whenever a value is used more than once.This way you are sure which value applies when.This is not often pretty, but is easier to visually verify.

The surprising case is doing this even for “final” fields.This is because JVMs are not always smart enough to exploit the fine points of the JMM and not reload read final values, as they would otherwise need to do across the volatile accesses entailed in locking. Some JVMs are smarter than they used to be about this, but still not always smart enough.

文章轉自 並發編程網-ifeve.com