如何从CERTIFICATE文件中抽取PUBLIC KEY

抽取前：
—–BEGIN CERTIFICATE—–
MIIDtjCCAx+gAwIBAgIQXIanT45PgITMoV2yUE9EbDANBgkqhkiG9w0BAQUFADAk
MQswCQYDVQQGEwJDTjEVMBMGA1UEChMMQ0ZDQSBURVNUIENBMB4XDTE0MDcwOTA4
MTMxM1oXDTE2MDcwMzA2NTUwM1owfDELMAkGA1UEBhMCQ04xFTATBgNVBAoTDENG
Q0EgVEVTVCBDQTENMAsGA1UECxMEVEVTVDESMBAGA1UECxMJQ3VzdG9tZXJzMTMw
MQYDVQQDFCowNDFAWjAzNTA1ODIxOTc5MDUzMTIwMTFAY25jYnRlc3RAMDAwMDAw
MDEwgZ8wDQYJKoZIhvcNAQEBBQADgY0AMIGJAoGBAMkBfqOtA1QPOSS6UEge/pdO
fRtegEF7WrwdCGNwP5FCIgNIfqNN99FVSOCASv234vzFkiF4/CgVbDiCf+lAg3Ys
MlSA9uCOW9ejg+Bhso6L09Qht771syFUrOX2jf38VuHLUCGPRyOc2exqRyjsvDvK
MKziZoJBk/wYI7iqtsHXAgMBAAGjggGPMIIBizAfBgNVHSMEGDAWgBRGctwlcp8C
TlWDtYD5C9vpk7P0RTAdBgNVHQ4EFgQUKxGjDYv6hBevitPVCIJHSiJroVIwCwYD
VR0PBAQDAgTwMAwGA1UdEwQFMAMBAQAwOwYDVR0lBDQwMgYIKwYBBQUHAwEGCCsG
AQUFBwMCBggrBgEFBQcDAwYIKwYBBQUHAwQGCCsGAQUFBwMIMIHwBgNVHR8Egegw
geUwT6BNoEukSTBHMQswCQYDVQQGEwJDTjEVMBMGA1UEChMMQ0ZDQSBURVNUIENB
MQwwCgYDVQQLEwNDUkwxEzARBgNVBAMTCmNybDEyNl8zMTIwgZGggY6ggYuGgYhs
ZGFwOi8vdGVzdGxkYXAuY2ZjYS5jb20uY246Mzg5L0NOPWNybDEyNl8zMTIsT1U9
Q1JMLE89Q0ZDQSBURVNUIENBLEM9Q04/Y2VydGlmaWNhdGVSZXZvY2F0aW9uTGlz
dD9iYXNlP29iamVjdGNsYXNzPWNSTERpc3RyaWJ1dGlvblBvaW50MA0GCSqGSIb3
DQEBBQUAA4GBAMUfsbhrAM0XDB3P1qppqa26qJCvOqngIT1DgAcZgVmLysJPvtji
fkJU4XgQVP3Hb5u3KcSyXqIip0kvpBOBbXVbc6S5/2AocNgr77sdGLdy/QQrHl7m
tbbqz8MS5b6T3S4hb7t3RTFdhH+vWHGzQSA5zBQpNztMq34qTZjfySWz
—–END CERTIFICATE—–

抽取后：
—–BEGIN PUBLIC KEY—–
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDJAX6jrQNUDzkkulBIHv6XTn0bXoBBe1q8HQhj
cD+RQiIDSH6jTffRVUjggEr9t+L8xZIhePwoFWw4gn/pQIN2LDJUgPbgjlvXo4PgYbKOi9PUIbe+
9bMhVKzl9o39/Fbhy1Ahj0cjnNnsakco7Lw7yjCs4maCQZP8GCO4qrbB1wIDAQAB
—–END PUBLIC KEY—–

代码如下：

    FileInputStream file = new FileInputStream("server.cer");
    CertificateFactory ft = CertificateFactory.getInstance("X.509");
    X509Certificate certificate = (X509Certificate)ft.generateCertificate(file);
    PublicKey publicKey = certificate.getPublicKey();
    BASE64Encoder b64 = new BASE64Encoder();
    System.out.println("-----BEGIN PUBLIC KEY-----");
    System.out.println(b64.encode(publicKey.getEncoded()) );
    System.out.println("-----END PUBLIC KEY-----");