一、从一个同步的Http调用说起

一个很简单的业务逻辑，其他后端服务提供了一个接口，我们需要通过接口调用，获取到响应的数据。

逆地理接口：通过经纬度获取这个经纬度所在的省市区县以及响应的code：

curl-i"http://xxx?latitude=31.08966221524924&channel=amap7a&near=false&longitude=105.13990312814713"

{"adcode":"510722"}

服务端执行，最简单的同步调用方式：

服务端响应之前，IO会阻塞在：
java.net.SocketInputStream#socketRead0 的native方法上：

通过jstack日志，可以发现，此时这个Thread会一直在runable的状态：

"main"#1 prio=5 os_prio=31 tid=0x00007fed0c810000 nid=0x1003 runnable [0x000070000ce14000]   java.lang.Thread.State: RUNNABLEat java.net.SocketInputStream.socketRead0(Native Method)at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)at java.net.SocketInputStream.read(SocketInputStream.java:171)at java.net.SocketInputStream.read(SocketInputStream.java:141)at org.apache.http.impl.conn.LoggingInputStream.read(LoggingInputStream.java:84)at org.apache.http.impl.io.SessionInputBufferImpl.streamRead(SessionInputBufferImpl.java:137)at org.apache.http.impl.io.SessionInputBufferImpl.fillBuffer(SessionInputBufferImpl.java:153)at org.apache.http.impl.io.SessionInputBufferImpl.readLine(SessionInputBufferImpl.java:282)at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:138)at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:56)at org.apache.http.impl.io.AbstractMessageParser.parse(AbstractMessageParser.java:259)at org.apache.http.impl.DefaultBHttpClientConnection.receiveResponseHeader(DefaultBHttpClientConnection.java:163)at org.apache.http.impl.conn.CPoolProxy.receiveResponseHeader(CPoolProxy.java:165)at org.apache.http.protocol.HttpRequestExecutor.doReceiveResponse(HttpRequestExecutor.java:273)at org.apache.http.protocol.HttpRequestExecutor.execute(HttpRequestExecutor.java:125)at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:272)at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:185)at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:89)at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:110)at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:185)at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:83)at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:108)at com.amap.aos.async.AsyncIO.blockingIO(AsyncIO.java:207).......

线程模型示例：

同步最大的问题是在IO等待的过程中，线程资源没有得到充分的利用，对于大量IO场景的业务吞吐量会有一定限制。

二、JDK NIO & Future

在JDK 1.5 中，JUC提供了Future抽象：

当然并不是所有的Future都是这样实现的，如
io.netty.util.concurrent.AbstractFuture 就是通过线程轮询去。

这样做的好处是，主线程可以不用等待IO响应，可以去做点其他的，比如说再发送一个IO请求，可以等到一起返回：

"main"#1 prio=5 os_prio=31 tid=0x00007fd7a500b000 nid=0xe03 waiting on condition [0x000070000a95d000]   java.lang.Thread.State: WAITING (parking)at sun.misc.Unsafe.park(Native Method)
- parking to wait for  <0x000000076ee2d768> (a java.util.concurrent.CountDownLatch$Sync)at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)at org.asynchttpclient.netty.NettyResponseFuture.get(NettyResponseFuture.java:162)at com.amap.aos.async.AsyncIO.futureBlockingGet(AsyncIO.java:201).....
"AsyncHttpClient-2-1"#11 prio=5 os_prio=31 tid=0x00007fd7a7247800 nid=0x340b runnable [0x000070000ba94000]   java.lang.Thread.State: RUNNABLEat sun.nio.ch.KQueueArrayWrapper.kevent0(Native Method)at sun.nio.ch.KQueueArrayWrapper.poll(KQueueArrayWrapper.java:198)at sun.nio.ch.KQueueSelectorImpl.doSelect(KQueueSelectorImpl.java:117)at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:86)
- locked <0x000000076eb00ef0> (a io.netty.channel.nio.SelectedSelectionKeySet)
- locked <0x000000076eb00f10> (a java.util.Collections$UnmodifiableSet)
- locked <0x000000076eb00ea0> (a sun.nio.ch.KQueueSelectorImpl)at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:97)at io.netty.channel.nio.NioEventLoop.select(NioEventLoop.java:693)at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:353)at io.netty.util.concurrent.SingleThreadEventExecutor$2.run(SingleThreadEventExecutor.java:140)at io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run(DefaultThreadFactory.java:144)at java.lang.Thread.run(Thread.java:748)