Keystore是一個密碼保護的文件，存放私鑰和證書。可以通過JDK自帶的keytool工具生成。

但是keytool工具，并沒有提供方便的方法，從keystore文件中到處私鑰和證書。

所以可以通過JDK提供的Java.security.KeyStore 類來編碼完成相關工作。

    import java.io.File;     
    import java.io.FileInputStream;     
    import java.io.FileWriter;     
    import java.security.Key;     
    import java.security.KeyPair;     
    import java.security.KeyStore;     
    import java.security.KeyStoreException;     
    import java.security.NoSuchAlgorithmException;     
    import java.security.PrivateKey;    
    import java.security.PublicKey;    
    import java.security.UnrecoverableKeyException;    
    import java.security.cert.Certificate;  
      
         
    import sun.misc.BASE64Encoder;   
      
    public class ExportPrivateKey {    
            private File keystoreFile;    
            private String keyStoreType;    
            private char[] password;    
            private String alias;    
            private File exportedFile;           
      
            public static KeyPair getPrivateKey(KeyStore keystore, String alias, char[] password) {    
                    try {    
                            Key key=keystore.getKey(alias,password);    
                            if(key instanceof PrivateKey) {    
                                    Certificate cert=keystore.getCertificate(alias);    
                                    PublicKey publicKey=cert.getPublicKey();    
                                    return new KeyPair(publicKey,(PrivateKey)key);    
                            }    
                    } catch (UnrecoverableKeyException e) {    
            } catch (NoSuchAlgorithmException e) {    
            } catch (KeyStoreException e) {    
            }    
            return null;    
            }  
      
       
      
            public void export() throws Exception{    
                    KeyStore keystore=KeyStore.getInstance(keyStoreType);    
                    BASE64Encoder encoder=new BASE64Encoder();    
                    keystore.load(new FileInputStream(keystoreFile),password);    
                    KeyPair keyPair=getPrivateKey(keystore,alias,password);    
                    PrivateKey privateKey=keyPair.getPrivate();    
                    String encoded=encoder.encode(privateKey.getEncoded());    
                    FileWriter fw=new FileWriter(exportedFile);    
                    fw.write(“—–BEGIN PRIVATE KEY—–/n“);    
                    fw.write(encoded);    
                    fw.write(“/n“);    
                    fw.write(“—–END PRIVATE KEY—–”);    
                    fw.close();  
      
            }  
      
       
            public static void main(String args[]) throws Exception{    
                    ExportPrivateKey export=new ExportPrivateKey();    
                    export.keystoreFile=new File(args[0]);    
                    export.keyStoreType=args[1];    
                    export.password=args[2].toCharArray();    
                    export.alias=args[3];    
                    export.exportedFile=new File(args[4]);    
                    export.export();    
            }    
    }

轉載：
http://blog.csdn.net/moreorless/article/details/4985940