当安卓客户端与服务器建立https连接时，需要验证服务器是否具备平台已知的CA证书。由于服务器的CA证书可能无法被安卓系统识别，有可能出现下面的异常:

javax.net.ssl.SSLHandshakeException: java.security.cert.CertPathValidatorException: Trust anchor for certification path not found.

谷歌官方也给出了这个问题的详细介绍，解决的方法是通过加载证书文件，创建一个KeyStore，用来初始化TrustManager，用来信任加载的CA证书。再通过SSLContext将这个证书集替换默认的sslSocketFactory。

今天提出一个简单的证书集工具类，可方便管理自己的证书集。

public class CustomTrustManager {
protected static X509TrustManager trustManager;
protected static SSLSocketFactory sslSocketFactory;

private CustomTrustManager() {
    try {
        //读取证书文件，可用字符串流或文件
        trustManager = trustManagerForCertificates(trustedCertificatesInputStream());
        SSLContext sslContext = SSLContext.getInstance("TLS");
        //使用sslContext将自定义证书集初始化
        sslContext.init(null, new TrustManager[]{trustManager}, null);
        sslSocketFactory = sslContext.getSocketFactory();
    } catch (GeneralSecurityException e) {
        throw new RuntimeException(e);
    }
}

//内部静态类的单例模式
private static class CustomTrustHolder {
    private static final CustomTrustManager INSTANCE = new CustomTrustManager();
}

public X509TrustManager getTrustManager(){
    return CustomTrustHolder.INSTANCE.trustManager;
}

public SSLSocketFactory getSSLSocketFactory(){
    return CustomTrustHolder.INSTANCE.sslSocketFactory;
}

public static CustomTrustManager getInstance() {
    return CustomTrustHolder.INSTANCE;
}

//用来创建证书和证书集，替换当前平台的证书集
//如果需要补授信某些服务器，可是使用CertificatePinner
private static X509TrustManager trustManagerForCertificates(InputStream in) throws
        GeneralSecurityException {
    CertificateFactory certificateFactory = CertificateFactory.getInstance("X.509");
    Collection<? extends Certificate> certificates = certificateFactory.generateCertificates
            (in);
    if (certificates.isEmpty()) {
        throw new IllegalArgumentException("expected non-empty set of trusted certificates");
    }

    //指定一个密码
    char[] password = "password".toCharArray();
    KeyStore keyStore = newEmptyKeyStore(password);
    int index = 0;
    for (Certificate certificate : certificates) {
        String certificateAlias = Integer.toString(index++);
        keyStore.setCertificateEntry(certificateAlias, certificate);
    }

    //初始化X509TrustManager
    KeyManagerFactory keyManagerFactory = KeyManagerFactory.getInstance(
            KeyManagerFactory.getDefaultAlgorithm());
    keyManagerFactory.init(keyStore, password);
    TrustManagerFactory trustManagerFactory = TrustManagerFactory.getInstance(
            TrustManagerFactory.getDefaultAlgorithm());
    trustManagerFactory.init(keyStore);
    TrustManager[] trustManagers = trustManagerFactory.getTrustManagers();
    if (trustManagers.length != 1 || !(trustManagers[0] instanceof X509TrustManager)) {
        throw new IllegalStateException("Unexpected default trust managers:"
                + Arrays.toString(trustManagers));
    }
    return (X509TrustManager) trustManagers[0];
}

private static KeyStore newEmptyKeyStore(char[] password) throws GeneralSecurityException {
    try {
        KeyStore keyStore = KeyStore.getInstance(KeyStore.getDefaultType());
        InputStream in = null; // By convention, 'null' creates an empty key store.
        keyStore.load(in, password);
        return keyStore;
    } catch (IOException e) {
        throw new AssertionError(e);
    }
}

//使用jdk的keytool命令读取证书内容 keytool -printcert -rfc -file 证书名.cer
private static InputStream trustedCertificatesInputStream() {
    //示例，此处替换从网站下载的CA证书
    String certA =
            "-----BEGIN CERTIFICATE-----\n" +
            “dLZzF2JaIn4LAmtQrFSM2sNRis\n" +
            "-----END CERTIFICATE-----";
    String certB = ...
    return new Buffer()
              .writeUtf8(CertA)
              //可指定多个证书
              .writeUtf8(CertB)
              .inputStream();
  }  
}  

在Chrome浏览器的开发者模式中，可以下载到https网站的证书。

1509959119(1).png

然后使用jdk的keytool命令复制证书内容，替换trustedCertificatesInputStream方法中的字符串。

1509959437(1).png

使用这个工具类，可以非常方便的使用自签名或者非认证的证书与服务器进行https连接。