org.jgroups.util.RingBuffer Maven / Gradle / Ivy
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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including
all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and
JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up
with different versions on classes on the class path).
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package org.jgroups.util;
import java.lang.reflect.Array;
import java.util.Arrays;
import java.util.Collection;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.function.BiConsumer;
/**
* Ring buffer of fixed capacity designed for multiple writers but only a single reader. Advancing the read or
* write index blocks until it is possible to do so.
* @author Bela Ban
* @since 4.0
*/
public class RingBuffer {
protected final T[] buf;
protected int ri, wi; // read and write indices
protected int count; // number of elements available to be read
protected final Lock lock=new ReentrantLock();
protected final Condition not_empty=lock.newCondition(); // reader can block on this
protected final Condition not_full=lock.newCondition(); // writes can block on this
public RingBuffer(Class element_type) {
buf=(T[])Array.newInstance(element_type, 16);
}
public RingBuffer(Class element_type, int capacity) {
int c=Util.getNextHigherPowerOfTwo(capacity); // power of 2 for faster mod operation
buf=(T[])Array.newInstance(element_type, c);
}
public T[] buf() {return buf;}
public int capacity() {return buf.length;}
public int readIndexLockless() {return ri;}
public int countLockLockless() {return count;}
public int readIndex() {
lock.lock();
try {
return ri;
}
finally {
lock.unlock();
}
}
public int writeIndex() {
lock.lock();
try {
return wi;
}
finally {
lock.unlock();
}
}
public int size() {
lock.lock();
try {
return count;
}
finally {
lock.unlock();
}
}
public boolean isEmpty() {
lock.lock();
try {
return ri == wi;
}
finally {
lock.unlock();
}
}
public RingBuffer clear() {
lock.lock();
try {
count=ri=wi=0;
Arrays.fill(buf, null);
return this;
}
finally {
lock.unlock();
}
}
/**
* Tries to add a new element at the current write index and advances the write index. If the write index is at the
* same position as the read index, this will block until the read index is advanced.
* @param element the element to be added. Must not be null, or else this operation returns immediately
*/
public RingBuffer put(T element) throws InterruptedException {
if(element == null)
return this;
lock.lock();
try {
while(count == buf.length)
not_full.await();
buf[wi]=element;
if(++wi == buf.length)
wi=0;
count++;
not_empty.signal();
return this;
}
finally {
lock.unlock();
}
}
/**
* Removes the next available element, blocking until one is available (if needed).
* @return The next available element
*/
public T take() throws InterruptedException {
lock.lock();
try {
while(count == 0)
not_empty.await();
T el=buf[ri];
buf[ri]=null;
if(++ri == buf.length)
ri=0;
count--;
not_full.signal();
return el;
}
finally {
lock.unlock();
}
}
/**
* Removes as many messages as possible and adds them to c.
* Same semantics as {@link java.util.concurrent.BlockingQueue#drainTo(Collection)}.
* @param c The collection to which to add the removed messages.
* @return The number of messages removed
* @throws NullPointerException If c is null
*/
public int drainTo(Collection c) {
return drainTo(c, Integer.MAX_VALUE);
}
/**
* Removes as many messages as possible and adds them to c. Contrary to {@link #drainTo(Collection)}, this method
* blocks until at least one message is available, or the caller thread is interrupted.
* @param c The collection to which to add the removed messages.
* @return The number of messages removed
* @throws NullPointerException If c is null
*/
public int drainToBlocking(Collection c) throws InterruptedException {
return drainToBlocking(c, Integer.MAX_VALUE);
}
/**
* Removes a number of messages and adds them to c.
* Same semantics as {@link java.util.concurrent.BlockingQueue#drainTo(Collection,int)}.
* @param c The collection to which to add the removed messages.
* @param max_elements The max number of messages to remove. The actual number of messages removed may be smaller
* if the buffer has fewer elements
* @return The number of messages removed
* @throws NullPointerException If c is null
*/
public int drainTo(Collection c, int max_elements) {
int num=Math.min(count, max_elements); // count may increase in the mean time, but that's ok
if(num == 0)
return num;
int read_index=ri; // no lock as we're the only reader
for(int i=0; i < num; i++) {
int real_index=realIndex(read_index +i);
c.add(buf[real_index]);
buf[real_index]=null;
}
publishReadIndex(num);
return num;
}
/**
* Removes a number of messages and adds them to c. Contrary to {@link #drainTo(Collection,int)}, this method
* blocks until at least one message is available, or the caller thread is interrupted.
* @param c The collection to which to add the removed messages.
* @param max_elements The max number of messages to remove. The actual number of messages removed may be smaller
* if the buffer has fewer elements
* @return The number of messages removed
* @throws NullPointerException If c is null
*/
public int drainToBlocking(Collection c, int max_elements) throws InterruptedException {
lock.lockInterruptibly(); // fail fast
try {
while(count == 0)
not_empty.await();
return drainTo(c, max_elements);
}
finally {
lock.unlock();
}
}
/**
* Removes messages and adds them to c.
* @param c The array to add messages to.
* @return The number of messages removed and added to c. This is min(count, c.length). If no messages are present,
* this method returns immediately
*/
public int drainTo(T[] c) {
int num=Math.min(count, c.length); // count may increase in the mean time, but that's ok
if(num == 0)
return num;
int read_index=ri; // no lock as we're the only reader
for(int i=0; i < num; i++) {
int real_index=realIndex(read_index +i);
c[i]=(buf[real_index]);
buf[real_index]=null;
}
publishReadIndex(num);
return num;
}
/**
* Removes messages and adds them to c.
* @param c The array to add messages to.
* @return The number of messages removed and added to c. This is min(count, c.length). Contrary to
* {@link #drainTo(Object[])}, this method blocks until at least one message is available or the caller thread
* is interrupted.
*/
public int drainToBlocking(T[] c) throws InterruptedException {
lock.lockInterruptibly(); // fail fast
try {
while(count == 0)
not_empty.await();
return drainTo(c);
}
finally {
lock.unlock();
}
}
public RingBuffer publishReadIndex(int num_elements_read) {
// this.ri is only read/written by the consumer, and since there's only 1 consumer, there's no need to synchronize on it
this.ri=realIndex(this.ri + num_elements_read);
lock.lock();
try {
this.count-=num_elements_read;
not_full.signalAll(); // wake up all writers
return this;
}
finally {
lock.unlock();
}
}
/** Blocks until messages are available */
public int waitForMessages() throws InterruptedException {
return waitForMessages(40, null);
}
/**
* Blocks until messages are available
* @param num_spins the number of times we should spin before acquiring a lock
* @param wait_strategy the strategy used to spin. The first parameter is the iteration count and the second
* parameter is the max number of spins
*/
public int waitForMessages(int num_spins, final BiConsumer wait_strategy) throws InterruptedException {
// try spinning first (experimental)
for(int i=0; i < num_spins && count == 0; i++) {
if(wait_strategy != null)
wait_strategy.accept(i, num_spins);
else
Thread.yield();
}
if(count == 0)
_waitForMessages();
return count; // whatever is the last count; could have been updated since lock release
}
public void _waitForMessages() throws InterruptedException {
lock.lockInterruptibly(); // fail fast
try {
while(count == 0)
not_empty.await();
}
finally {
lock.unlock();
}
}
public String toString() {
return String.format("[ri=%d wi=%d size=%d cap=%d]", ri, wi, size(), buf.length);
}
/** Apparently much more efficient than mod (%) */
protected int realIndex(int index) {
return index & (buf.length -1);
}
}