How to export private key from BIGIP?

Here is sample code to generate key and print key as pem.

ManagementKeyCertificateBindingStub stub = interfaces.getManagementKeyCertificate();
interfaces.getManagementPartition().set_active_partition("Common");

ManagementKeyCertificateManagementModeType mode = ManagementKeyCertificateManagementModeType.MANAGEMENT_MODE_DEFAULT;

ManagementKeyCertificateKey[] keys = new ManagementKeyCertificateKey[1];
ManagementKeyCertificateX509Data[] x509_datas = new ManagementKeyCertificateX509Data[1];
String key_id = "testkey001";

ManagementKeyCertificateKey key = new ManagementKeyCertificateKey();
key.setId(key_id);
key.setKey_type(ManagementKeyCertificateKeyType.KTYPE_RSA_PRIVATE);
key.setBit_length(2048L);
key.setSecurity(ManagementKeyCertificateSecurityType.STYPE_NORMAL);
keys[0] = key;

ManagementKeyCertificateX509Data x509_data = new ManagementKeyCertificateX509Data();
x509_data.setCommon_name("www.hogehoge.hoge");
x509_data.setCountry_name("JP");
x509_data.setDivision_name("hoge");
x509_data.setLocality_name("hoge");
x509_data.setOrganization_name("hoge");
x509_data.setState_name("Tokyo");
x509_datas[0] = x509_data;

boolean create_optional_cert_csr = true;
boolean overwrite = false;

stub.key_generate(mode, keys, x509_datas, create_optional_cert_csr, overwrite);

String[] key_ids = {key_id};
System.out.println(stub.key_export_to_pem(mode,key_ids)[0]);

also, you can find generated key on

/config/filestore/files_d/Common_d/certificate_key_d/:Common:testkey001.key_1148889_1