String content = "";
for(int i=1;i<100;i++)content += i + ") Easy to say, hard to do.\n";

/*1、利用公钥加密至文件*/
{
    /*1）根据公钥反向构造PublickKey对象，调用cipher的init方法时使用*/
    String KEY_PUBLIC  = "MFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAIv38xXk06As11OCWq0IPQoQAmz20ZiE8T5KeaMTUbhzUkGNTAQygApua71R/INeEDNsyyQS4PT6EaWTjJop2rcCAwEAAQ==";
    X509EncodedKeySpec keySpec = new X509EncodedKeySpec(new BASE64Decoder().decodeBuffer(KEY_PUBLIC));
    PublicKey key = KeyFactory.getInstance("RSA").generatePublic(keySpec);
    
    Cipher cipher = Cipher.getInstance("RSA");//RSA/ECB/PKCS1PADDING
    cipher.init(Cipher.ENCRYPT_MODE, key);

    /*
     * 直接加密，因为内容太长将有异常，需要分块加密
     * javax.crypto.IllegalBlockSizeException: Data must not be longer than 53 bytes
     * cipher.doFinal(content.getBytes());
     */
    
    /*2）分块加密，与KeyPairGenerator.initialize(int keysize)有关，本例使用的keysize为512*/
    int blockSize = 53;//cipher.getBlockSize()为0？根据异常提示设的53
    byte[] encryptByte = content.getBytes();
    //根据块大小分块，不足一块的部分为一块
    int blocksNum = (int)Math.ceil((1.0*encryptByte.length)/blockSize);
    
    FileOutputStream out = new FileOutputStream("license.lic");
    for (int i = 0; i < blocksNum; i++) {
        if (i < blocksNum - 1) {
            out.write(cipher.doFinal(encryptByte, i * blockSize, blockSize));
        } else {
            out.write(cipher.doFinal(encryptByte, i * blockSize, encryptByte.length - i * blockSize));
        }
    }
    
    out.close();
}

/*2、利用私钥从文件解密*/
{
    /*1）根据私钥反向构造PrivateKey对象*/
    String KEY_PRIVATE = "MIIBVAIBADANBgkqhkiG9w0BAQEFAASCAT4wggE6AgEAAkEAi/fzFeTToCzXU4JarQg9ChACbPbRmITxPkp5oxNRuHNSQY1MBDKACm5rvVH8g14QM2zLJBLg9PoRpZOMminatwIDAQABAkBqDAR7FBJbC15hgSQecV194D9WO3L18dOt9FNQgPSroGVYIvIizp1/wIHpMTN6uHwSoaZQcOqV33gxLF6fKbwxAiEAv390Q3X2cBjeScxhNbqPiOspE9rYD3eWSajN6Q7ud7UCIQC7HTUZelHMCpv4xPzg6e1QZkWhBfuqkhg9aOeAnIW0OwIgLW5Tat3FhXqg4ek29sQ34UfJCwjUUXcRlJATqcL9GDECIQChqe+JzrxDbVsrCY9vB83JLEO2hwPUcJtO24dBAHsopwIgEV547YcgZ+pyI1dnQhiLJiiFif+h1aBzaIH5mrkshtw=";
    PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(new BASE64Decoder().decodeBuffer(KEY_PRIVATE));
    PrivateKey key = KeyFactory.getInstance("RSA").generatePrivate(keySpec);
    
    Cipher cipher = Cipher.getInstance("RSA");//RSA/ECB/PKCS1PADDING
    cipher.init(Cipher.DECRYPT_MODE, key);
    
    /*2）分块解密，根据每块加密输出大小确定*/
    FileInputStream cis = new FileInputStream("license.lic");
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    /*加密时分块加密，加密输出结果大小固定*/
    byte[] data = new byte[cipher.getOutputSize(cis.available())];
    int len = 0;
    while((len = cis.read(data)) > 0){
        bos.write(cipher.doFinal(data, 0, len));
    }
    cis.close();
    
    System.out.print(new String(bos.toByteArray()));
}

RSA非对称加密解密实例