最近在做一個定時任務，每一個小時使用HttpClient去訪問一個外部伺服器拉去一組資料，使用一個大小為10的執行緒池啟動拉取執行緒，昨天外部伺服器掛了一次，然後發現我自己的伺服器上的執行緒池也掛了，提交的新task全部無法執行。奇怪的是外部伺服器昨天掛了，今天就恢復了，我自己的伺服器今天應該也自動恢復才對啊，我重啟了自己的伺服器的程序就OK了，但為什麼執行緒池會掛呢，即使外部服務恢復了，本地執行緒池必須要重啟來才能恢復，這是為何？

本地寫了一個小測試程式，自己搭建了一個簡單的http server，在server的服務方法中啟動死迴圈（或者直接打斷點不退出），就發現client端的請求執行緒一直卡住不會釋放，如果這個是執行緒池中的執行緒，就會一直佔用執行緒池資源，導致執行緒池不能響應後續的的任務。

解決辦法，設定socket超時時間：

SSLContextBuilder builder = new SSLContextBuilder();
            builder.loadTrustMaterial(null, new TrustSelfSignedStrategy());
            SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(
                    builder.build());

            RequestConfig requestConfig = RequestConfig.custom
 
().setSocketTimeout(5000)
                    .setConnectTimeout(5000).setConnectionRequestTimeout(5000).build();

            CloseableHttpClient client = HttpClientBuilder.create().setMaxConnTotal(3)
                    .setMaxConnPerRoute(10000).setSSLSocketFactory(sslsf).build();

            // URI uri = UriBuilder.fromPath
 
("http://localhost:8080/")
            URI uri = UriBuilder.fromPath("https://localhost:8443/")
                    .segment("hello-world")
                    .segment("get")
                    .build();
            HttpGet get = new HttpGet(uri);
            get.setConfig(requestConfig);
            System.out.println("http start");
            try {
                CloseableHttpResponse response = client.execute(get);
            } catch (Exception e) {
                e.printStackTrace();
            }
            System.out.println("http down");
            return "ok";

這個時候執行緒就不會一直卡住了，達到超時時間就會會丟擲異常，並釋放執行緒資源：

java.net.SocketTimeoutException: Read timed out
    at java.net.SocketInputStream.socketRead0(Native Method)
    at java.net.SocketInputStream.socketRead(SocketInputStream.java:116)
    at java.net.SocketInputStream.read(SocketInputStream.java:170)
    at java.net.SocketInputStream.read(SocketInputStream.java:141)
    at sun.security.ssl.InputRecord.readFully(InputRecord.java:465)
    at sun.security.ssl.InputRecord.read(InputRecord.java:503)
    at sun.security.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:973)
    at sun.security.ssl.SSLSocketImpl.readDataRecord(SSLSocketImpl.java:930)
    at sun.security.ssl.AppInputStream.read(AppInputStream.java:105)
    at org.apache.http.impl.conn.LoggingInputStream.read(LoggingInputStream.java:87)
    at org.apache.http.impl.io.SessionInputBufferImpl.streamRead(SessionInputBufferImpl.java:136)
    at org.apache.http.impl.io.SessionInputBufferImpl.fillBuffer(SessionInputBufferImpl.java:152)
    at org.apache.http.impl.io.SessionInputBufferImpl.readLine(SessionInputBufferImpl.java:270)
    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:140)
    at org.apache.http.impl.conn.DefaultHttpResponseParser.parseHead(DefaultHttpResponseParser.java:57)
    at org.apache.http.impl.io.AbstractMessageParser.parse(AbstractMessageParser.java:260)
    at org.apache.http.impl.DefaultBHttpClientConnection.receiveResponseHeader(DefaultBHttpClientConnection.java:161)
    at org.apache.http.impl.conn.CPoolProxy.receiveResponseHeader(CPoolProxy.java:153)
    at org.apache.http.protocol.HttpRequestExecutor.doReceiveResponse(HttpRequestExecutor.java:271)
    at org.apache.http.protocol.HttpRequestExecutor.execute(HttpRequestExecutor.java:123)
    at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:254)
    at org.apache.http.impl.execchain.ProtocolExec.execute(ProtocolExec.java:195)
    at org.apache.http.impl.execchain.RetryExec.execute(RetryExec.java:86)
    at org.apache.http.impl.execchain.RedirectExec.execute(RedirectExec.java:108)
    at org.apache.http.impl.client.InternalHttpClient.doExecute(InternalHttpClient.java:184)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:82)
    at org.apache.http.impl.client.CloseableHttpClient.execute(CloseableHttpClient.java:106)
    at dropwizardDemo.client.TestHttpClient$TestCallable.call(TestHttpClient.java:64)
    at dropwizardDemo.client.TestHttpClient$TestCallable.call(TestHttpClient.java:1)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

可見丟擲這個異常的方法已經是native方法了