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使用httpclient对接多个第三方应用，云服务形式的接口都没有问题，唯独一家私有部署的系统，死活握不了手。对方的系统用浏览器访问无误（https）。程序访问就是不行：
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:1747)
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:1209)
at com.sun.net.ssl.internal.ssl.ClientHandshaker.processMessage(ClientHandshaker.java:135)
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:943)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.performInitialHandshake(SSLSocketImpl.java:1188)
at com.sun.net.ssl.internal.ssl.SSLSocketImpl.writeRecord(SSLSocketImpl.java:654)
at com.sun.net.ssl.internal.ssl.AppOutputStream.write(AppOutputStream.java:100)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
at org.apache.commons.httpclient.methods.StringRequestEntity.writeRequest(StringRequestEntity.java:150)
at org.apache.commons.httpclient.methods.EntityEnclosingMethod.writeRequestBody(EntityEnclosingMethod.java:495)
at org.apache.commons.httpclient.HttpMethodBase.writeRequest(HttpMethodBase.java:1973)
at org.apache.commons.httpclient.HttpMethodBase.execute(HttpMethodBase.java:993)
at org.apache.commons.httpclient.HttpMethodDirector.executeWithRetry(HttpMethodDirector.java:397)
at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:170)
at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:396)
at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:324)
奇怪的是同一个程序访问其他家的https接口就没有。
其实核心原因很简单，就是java的自身证书库里没有他们家的。按如下步骤操作即可解决问题
1、找对方要证书，例如server.cer
2、复制到$JAVA_HOME/jre/lib/security/目录下
3、执行keytool -import -alias mycert -file ./server.cer -keystore ./cacerts -storepass changeit
其中changeit是java证书文件默认访问密码，如果改过就需要搜索自己的脑子了。
操作后，重启程序，问题OK