请求request_token出现如下异常，求解
- Java code
11-08 15:08:43.516: W/System.err(2770): oauth.signpost.exception.OAuthCommunicationException: Communication with the service provider failed: Not trusted server certificate11-08 15:08:43.525: W/System.err(2770): at oauth.signpost.AbstractOAuthProvider.retrieveToken(AbstractOAuthProvider.java:215)11-08 15:08:43.525: W/System.err(2770): at oauth.signpost.AbstractOAuthProvider.retrieveRequestToken(AbstractOAuthProvider.java:69)11-08 15:08:43.525: W/System.err(2770): at com.mypack.oauth.OAuthAsyncTask.doInBackground(OAuthAsyncTask.java:27)11-08 15:08:43.525: W/System.err(2770): at com.mypack.oauth.OAuthAsyncTask.doInBackground(OAuthAsyncTask.java:1)11-08 15:08:43.536: W/System.err(2770): at android.os.AsyncTask$2.call(AsyncTask.java:185)11-08 15:08:43.546: W/System.err(2770): at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)11-08 15:08:43.546: W/System.err(2770): at java.util.concurrent.FutureTask.run(FutureTask.java:137)11-08 15:08:43.556: W/System.err(2770): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1068)11-08 15:08:43.556: W/System.err(2770): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:561)11-08 15:08:43.566: W/System.err(2770): at java.lang.Thread.run(Thread.java:1096)11-08 15:08:43.566: W/System.err(2770): Caused by: javax.net.ssl.SSLException: Not trusted server certificate11-08 15:08:43.566: W/System.err(2770): at org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:371)11-08 15:08:43.566: W/System.err(2770): at org.apache.http.conn.ssl.AbstractVerifier.verify(AbstractVerifier.java:92)11-08 15:08:43.576: W/System.err(2770): at org.apache.http.conn.ssl.SSLSocketFactory.createSocket(SSLSocketFactory.java:381)11-08 15:08:43.586: W/System.err(2770): at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:164)11-08 15:08:43.595: W/System.err(2770): at org.apache.http.impl.conn.AbstractPoolEntry.open(AbstractPoolEntry.java:164)11-08 15:08:43.606: W/System.err(2770): at org.apache.http.impl.conn.AbstractPooledConnAdapter.open(AbstractPooledConnAdapter.java:119)11-08 15:08:43.616: W/System.err(2770): at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:348)11-08 15:08:43.616: W/System.err(2770): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:555)11-08 15:08:43.616: W/System.err(2770): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:487)11-08 15:08:43.637: W/System.err(2770): at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:465)11-08 15:08:43.637: W/System.err(2770): at oauth.signpost.commonshttp.CommonsHttpOAuthProvider.sendRequest(CommonsHttpOAuthProvider.java:92)11-08 15:08:43.686: W/System.err(2770): at oauth.signpost.AbstractOAuthProvider.retrieveToken(AbstractOAuthProvider.java:178)11-08 15:08:43.686: W/System.err(2770): ... 9 more11-08 15:08:43.686: W/System.err(2770): Caused by: java.security.cert.CertificateException: java.security.cert.CertPathValidatorException: Could not validate certificate signature.11-08 15:08:43.696: W/System.err(2770): at org.apache.harmony.xnet.provider.jsse.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:168)11-08 15:08:43.706: W/System.err(2770): at org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:366)11-08 15:08:43.706: W/System.err(2770): ... 20 more11-08 15:08:43.706: W/System.err(2770): Caused by: java.security.cert.CertPathValidatorException: Could not validate certificate signature.11-08 15:08:43.726: W/System.err(2770): at org.bouncycastle.jce.provider.PKIXCertPathValidatorSpi.engineValidate(PKIXCertPathValidatorSpi.java:342)11-08 15:08:43.726: W/System.err(2770): at java.security.cert.CertPathValidator.validate(CertPathValidator.java:202)11-08 15:08:43.726: W/System.err(2770): at org.apache.harmony.xnet.provider.jsse.TrustManagerImpl.checkServerTrusted(TrustManagerImpl.java:164)11-08 15:08:43.726: W/System.err(2770): ... 21 more11-08 15:08:43.737: W/System.err(2770): Caused by: java.security.SignatureException: Signature was not verified.11-08 15:08:43.756: W/System.err(2770): at org.apache.harmony.security.provider.cert.X509CertImpl.fastVerify(X509CertImpl.java:601)11-08 15:08:43.756: W/System.err(2770): at org.apache.harmony.security.provider.cert.X509CertImpl.verify(X509CertImpl.java:544)11-08 15:08:43.766: W/System.err(2770): at org.bouncycastle.jce.provider.PKIXCertPathValidatorSpi.engineValidate(PKIXCertPathValidatorSpi.java:337)11-08 15:08:43.766: W/System.err(2770): ... 23 more