[英]How to build a recursive tree in Java-ee?
Here is my pseudo-code: 这是我的伪代码:
class Builder implements Callable<T> {
T obj;
ManagedExecutorService pool;
Builder (T obj, ManagedExecutorService pool){
this.obj = obj;
this.pool = pool;
}
T call(){
build();
}
private void build(){
// skip if already traversed
return isTraversed(obj);
// call db and get obj's one-to-many relationships
Collection<T> colOfChildObj = DbUtil.getChildrenPOJO(obj);
for (<Collection>T childObj : colOfChildObj){
this.pool.submit(new Builder(childObj, this.pool));
}
// set children as-is, when the submit above completes,
// it will update childObj and thus will reflect
// obj.childObj.itsChidren etc. For this though the caller
// has to wait until all submits are processed
obj.setChildren(colOfChildObj);
}
}
Since Java-ee does not support ForkJoinPool - that is out of the question. 由于Java-ee不支持ForkJoinPool-这是不可能的。 So how do I do it with either ManagedThreadFactory and/or ManagedExecutorService? 那么,如何使用ManagedThreadFactory和/或ManagedExecutorService呢? My real challenge is due to not being able to call pool.shutdown() or pool.awaitTermination in Java-ee. 我真正的挑战是由于无法在Java-ee中调用pool.shutdown()或pool.awaitTermination。 So, from the caller, if I do: 因此,从呼叫者那里,如果我这样做:
class Caller () {
T getObjGraph(T rootObj){
pool.submit(new Builder(rootObj));
T objGraph = pool.get();
return objGraph;
}
}
Then my method does not wait for all the pool.submit(new Builder(childObj, pool)) and thus my object does not have everything set and is incomplete. 然后,我的方法不会等待所有pool.submit(new Builder(childObj,pool)),因此我的对象没有设置所有内容,并且不完整。 I thought of putting all Futures returned by pool.submit into a blocking queue - but then I don't know how to inform the caller that my tree traversal is complete. 我曾考虑将pool.submit返回的所有Futures放入阻塞队列中-但是后来我不知道如何通知调用者我的树遍历已完成。 I do have a counter that reaches 0 when the tree traversal is complete but since the client is submitting a top level node, I'm not sure how to make it wait there in Java-ee without while(isCounter = 0) - which is a CPU hog. 树遍历完成后,我确实有一个达到0的计数器,但是由于客户端正在提交顶级节点,因此我不确定如何在没有while(isCounter = 0)的情况下在Java-ee中等待它的到达- CPU猪。
Any pointers? 有指针吗?
I think I understand what you're trying to do. 我想我了解您正在尝试做的事情。 You can just use a thread-safe counter, increment it every time you create and submit a new task for a given node, and decrement it when the task for this node is terminated. 您可以只使用线程安全计数器,在每次创建并提交给定节点的新任务时将其递增,并在终止该节点的任务时将其递减。
In the main thread, you wait on a lock util the remaining number of nodes to process is 0. And in each task, you notify the lock to signal that a tack is terminated. 在主线程中,您等待一个锁util,剩余要处理的节点数为0。在每个任务中,您通知锁以信号表示平头钉已终止。
Here is a complete example. 这是一个完整的例子。 It starts from a tree where each node has a name, and transforms this tree into another tree where each node is "Hello " concatenated with the original name. 它从一棵树开始,其中每个节点都有一个名称,然后将该树转换成另一棵树,其中每个节点都与原始名称串联在一起。
public class Tree {
public static void main(String[] args) throws ExecutionException, InterruptedException {
Node root = new Node("R");
Node c1 = new Node("C1");
Node c2 = new Node("C2");
root.addChild(c1);
root.addChild(c2);
Node gc11 = new Node("GC11");
Node gc12 = new Node("GC12");
c1.addChild(gc11);
c1.addChild(gc12);
Node gc21 = new Node("GC11");
Node gc22 = new Node("GC12");
c2.addChild(gc21);
c2.addChild(gc22);
System.out.println("root = " + root);
ExecutorService executor = Executors.newFixedThreadPool(4);
final Object lock = new Object();
final AtomicInteger remaining = new AtomicInteger(0);
Future<Node> result = executor.submit(new HelloTask(root, null, executor, remaining, lock));
synchronized (lock) {
while (remaining.get() != 0) {
lock.wait();
}
}
Node helloRoot = result.get();
System.out.println("helloRoot = " + helloRoot);
executor.shutdown();
}
private static class HelloTask implements Callable<Node> {
private final Node source;
private final Node parent;
private final ExecutorService executorService;
private final Object lock;
private final AtomicInteger remaining;
public HelloTask(Node source, Node parent, ExecutorService executorService, AtomicInteger remaining, Object lock) {
this.source = source;
this.parent = parent;
this.executorService = executorService;
this.lock = lock;
this.remaining = remaining;
remaining.incrementAndGet();
}
@Override
public Node call() throws Exception {
// simulate some time
Thread.sleep(1000L);
Node result = new Node("Hello " + source.getName());
if (parent != null) {
parent.addChild(result);
}
for (Node child : source.getChildren()) {
executorService.submit(new HelloTask(child, result, executorService, remaining, lock));
}
remaining.decrementAndGet();
synchronized (lock) {
lock.notifyAll();
}
return result;
}
}
private static class Node {
private final String name;
private final List<Node> children = new CopyOnWriteArrayList<>();
public Node(String name) {
this.name = name;
}
public String getName() {
return name;
}
public List<Node> getChildren() {
return children;
}
public void addChild(Node child) {
this.children.add(child);
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append(name);
sb.append('\n');
children.forEach(sb::append);
return sb.toString();
}
}
}
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