Read / Write Locks in Java | tutorials.jenkov.com
A read / write lock is more sophisticated lock than the Lock implementations shown in the text Locks in Java. Imagine you have an application that reads and writes some resource, but writing it is not done as much as reading it is. Two threads reading the same resource does not cause problems for each other, so multiple threads that want to read the resource are granted access at the same time, overlapping. But, if a single thread wants to write to the resource, no other reads nor writes must be in progress at the same time. To solve this problem of allowing multiple readers but only one writer, you will need a read / write lock.
Java 5 comes with read / write lock implementations in the java.util.concurrent package. Even so, it may still be useful to know the theory behind their implementation.
Read / Write Lock Java Implementation
First let's summarize the conditions for getting read and write access to the resource:
| Read Access | If no threads are writing, and no threads have requested write access. |
| Write Access | If no threads are reading or writing. |
If a thread wants to read the resource, it is okay as long as no threads are writing to it, and no threads have requested write access to the resource. By up-prioritizing write-access requests we assume that write requests are more important than read-requests. Besides, if reads are what happens most often, and we did not up-prioritize writes, starvation could occur. Threads requesting write access would be blocked until all readers had unlocked the ReadWriteLock. If new threads were constantly granted read access the thread waiting for write access would remain blocked indefinately, resulting in starvation. Therefore a thread can only be granted read access if no thread has currently locked the ReadWriteLock for writing, or requested it locked for writing.
A thread that wants write access to the resource can be granted so when no threads are reading nor writing to the resource. It doesn't matter how many threads have requested write access or in what sequence, unless you want to guarantee fairness between threads requesting write access.
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