Java模拟JavaScript生成器函数

js中可以定义生成器函数, 使用yield迭代结果, 例如

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function* gen() { 
yield 1;
yield 2;
yield 3;
}

let g = gen();
g.next();
g.next();

本文中尝试在Java环境中实现类似的效果

实现

Generator.java

生成器类

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import java.util.concurrent.Semaphore;

public class Generator {

private Function function;

private Runnable runnable;
private Thread thread;

private Semaphore readLock;
private Semaphore writeLock;

private Result result;
private Object nextValue;
private boolean done = true;

public Generator(Function function) {
this.function = function;
this.runnable = () -> {
try {
this.writeLock.acquire();
Object value = this.function.run(this);
this.result = new Result(value, true);
this.readLock.release();
} catch (Exception e) {
e.printStackTrace();
}
};
this.reset();
}

public boolean isDone() {
return done;
}

public synchronized void reset() {
this.close();
this.readLock = new Semaphore(0);
this.writeLock = new Semaphore(0);
this.result = null;
this.nextValue = null;
this.done = false;
this.thread = new Thread(this.runnable);
this.thread.start();
}

public synchronized void close() {
if (this.thread != null) {
this.thread.stop();
this.thread = null;
}
}

public synchronized Result next() throws InterruptedException {
return this.next(null);
}

public synchronized Result next(Object value) throws InterruptedException {
if (this.done) {
return null;
}
this.nextValue = value;
this.writeLock.release();
this.readLock.acquire();
Result result = this.result;
this.done = result.isDone();
return result;
}

public Object yield(Object value) throws InterruptedException {
this.result = new Result(value, false);
this.readLock.release();
this.writeLock.acquire();
return this.nextValue;
}

@FunctionalInterface
public interface Function {
Object run(Generator context) throws Exception;
}

public static class Result {
private Object value;
private boolean done;

private Result(Object value, boolean done) {
this.value = value;
this.done = done;
}

public Object getValue() {
return value;
}

public boolean isDone() {
return done;
}

@Override
public String toString() {
return "Result{" +
"value=" + value +
", done=" + done +
'}';
}
}
}

测试

Java

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public class Main {

public static void main(String[] args) throws InterruptedException {
Generator generator = new Generator(context -> {
int count = 0;
for (int i = 0; i < 10; i++) {
Integer value = (Integer) context.yield(i);
count += value == null ? 0 : value;
}
return count;
});

Generator.Result result = null;
do {
result = generator.next(result == null ? null : result.getValue());
System.out.println(result);
} while (!result.isDone());
}
}

/* 输出结果
Result{value=0, done=false}
Result{value=1, done=false}
Result{value=2, done=false}
Result{value=3, done=false}
Result{value=4, done=false}
Result{value=5, done=false}
Result{value=6, done=false}
Result{value=7, done=false}
Result{value=8, done=false}
Result{value=9, done=false}
Result{value=45, done=true}
*/

对应的JavaScript代码

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let generator = (function* Generator() {
let count = 0;
for (let i = 0; i < 10; i++) {
count += (yield i) || 0;
}
return count;
})();

let result;
do {
result = generator.next(result && result.value);
console.log(result);
} while (!result.done)

/* 输出结果
{value: 0, done: false}
{value: 1, done: false}
{value: 2, done: false}
{value: 3, done: false}
{value: 4, done: false}
{value: 5, done: false}
{value: 6, done: false}
{value: 7, done: false}
{value: 8, done: false}
{value: 9, done: false}
{value: 45, done: true}
*/
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