Try many ways,but can't solve sun.security.validator.ValidatorException: PKIX path building failed

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Java

security

HTTPS

Hi,all
We use apache's httpclient to post a request via https,when we test the code in the local machine or testing machine,it is all ok.But when we deploy the code in the product machine,when posting the request via https,we get such an exception

javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
        at com.sun.net.ssl.internal.ssl.Alerts.getSSLException(Alerts.java:174)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.fatal(SSLSocketImpl.java:1731)
        at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:241)
        at com.sun.net.ssl.internal.ssl.Handshaker.fatalSE(Handshaker.java:235)
        at com.sun.net.ssl.internal.ssl.ClientHandshaker.serverCertificate(ClientHandshaker.java:1206)
        at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:136)
        at com.sun.net.ssl.internal.ssl.Handshaker.processLoop(Handshaker.java:593)
        at com.sun.net.ssl.internal.ssl.Handshaker.process_record(Handshaker.java:529)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.readRecord(SSLSocketImpl.java:925)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1170)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1197)
        at com.sun.net.ssl.internal.ssl.SSLSocketImpl.startHandshake(SSLSocketImpl.java:1181)
        at org.apache.http.conn.ssl.SSLConnectionSocketFactory.createLayeredSocket(SSLConnectionSocketFactory.java:275)
        at org.apache.http.conn.ssl.SSLConnectionSocketFactory.connectSocket(SSLConnectionSocketFactory.java:254)
        at org.apache.http.impl.conn.HttpClientConnectionOperator.connect(HttpClientConnectionOperator.java:123)
        at org.apache.http.impl.conn.PoolingHttpClientConnectionManager.connect(PoolingHttpClientConnectionManager.java:318)
        at org.apache.http.impl.execchain.MainClientExec.establishRoute(MainClientExec.java:363)
        at org.apache.http.impl.execchain.MainClientExec.execute(MainClientExec.java:219)
        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 com.wechatpay.v3.common.HttpsRequest.sendPost(HttpsRequest.java:140)

then I follow this link http://www.java-samples.com/showtutorial.php?tutorialid=210[^] , using the browser to export the target server's certificate,and then I import it to java's keystore(jre/lib/security/cacerts) . After restarting, It's ok now.But after a while,the same exception raises again,and we restart the tomcat again,and now it's ok again,and after a while,the same problem..........

we use jdk6 and tomcat6.

Could anyone help me figure out this strange problem?It really drove me crazy!
Best regards.