在android app端访问https服务时，搜到的资料大都是自签名证书认证的逻辑。想办法扒到了Google官方的资料看了一下，豁然开朗。
基础概念就不解释了，只说使用方法。
使用CA签署的证书
如果服务端使用的是知名CA机构签名的证书，android连接https的代码非常简单：
URL url = new URL("https://wikipedia.org");
URLConnection urlConnection = url.openConnection();
InputStream in = urlConnection.getInputStream();
copyInputStreamToOutputStream(in, System.out);
常见问题
如果在链接https时遇到错误：
javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.
at org.apache.harmony.xnet.provider.jsse.OpenSSLSocketImpl.startHandshake(OpenSSLSocketImpl.java:374)
at libcore.net.http.HttpConnection.setupSecureSocket(HttpConnection.java:209)
at libcore.net.http.HttpsURLConnectionImpl$HttpsEngine.makeSslConnection(HttpsURLConnectionImpl.java:478)
at libcore.net.http.HttpsURLConnectionImpl$HttpsEngine.connect(HttpsURLConnectionImpl.java:433)
at libcore.net.http.HttpEngine.sendSocketRequest(HttpEngine.java:290)
at libcore.net.http.HttpEngine.sendRequest(HttpEngine.java:240)
at libcore.net.http.HttpURLConnectionImpl.getResponse(HttpURLConnectionImpl.java:282)
at libcore.net.http.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:177)
at libcore.net.http.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:271)
可能有以下几个原因：
1.签名证书的CA机构不知名
2.使用了自签名的证书
3.服务端配置问题
解决方案
1.导致SSLHandshakeException是由于签署证书的CA不被系统所信任。一种原因是CA机构比较新，还没被android系统证书库内置。或者你的android版本比较旧，证书库不全。
这种情况的解决方案和使用了自签名证书的方案是一样的，重写Google默认的证书校验逻辑。
// Load CAs from an InputStream
// (could be from a resource or ByteArrayInputStream or ...)
CertificateFactory cf = CertificateFactory.getInstance("X.509");
// From https://www.washington.edu/itconnect/security/ca/load-der.crt
InputStream caInput = new BufferedInputStream(new FileInputStream("load-der.crt"));
Certificate ca;
try {
ca = cf.generateCertificate(caInput);
System.out.println("ca=" + ((X509Certificate) ca).getSubjectDN());
} finally {
caInput.close();
}
// Create a KeyStore containing our trusted CAsString keyStoreType = KeyStore.getDefaultType();
KeyStore keyStore = KeyStore.getInstance(keyStoreType);
keyStore.load(null, null);
keyStore.setCertificateEntry("ca", ca);
// Create a TrustManager that trusts the CAs in our KeyStore
String tmfAlgorithm = TrustManagerFactory.getDefaultAlgorithm();
TrustManagerFactory tmf = TrustManagerFactory.getInstance(tmfAlgorithm);
tmf.init(keyStore);
// Create an SSLContext that uses our TrustManager
SSLContext context = SSLContext.getInstance("TLS");
context.init(null, tmf.getTrustManagers(), null);
// Tell the URLConnection to use a SocketFactory from our SSLContextURL url = new URL("https://certs.cac.washington.edu/CAtest/");
HttpsURLConnection urlConnection = (HttpsURLConnection)url.openConnection();
urlConnection.setSSLSocketFactory(context.getSocketFactory());
InputStream in = urlConnection.getInputStream();
copyInputStreamToOutputStream(in, System.out);
2.服务器配置问题
修改服务器配置或者使用上面的解决方案，直接信任该证书
