/// danger

Never store user's plaintext passwords. Always store a "secure hash" that you can then verify.

If you don't know, you will learn what a "password hash" is in the [security chapters](security/simple-oauth2.md#password-hashing){.internal-link target=_blank}.

///

## Multiple models { #multiple-models }

* Memory
* Previous steps before starting

    // in combination with immutable cache entries, will ensure a thread-safe cache.
    CacheEntry<K, V> entry;

    // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
    entry = cacheEntry1;
    if (entry != null && entry.key == key) {
      return entry.value;
    }
    entry = cacheEntry2;
    if (entry != null && entry.key == key) {

      .addInterceptor(new GzipRequestInterceptor())
      .build();
  private final Moshi moshi = new Moshi.Builder().build();
  private final JsonAdapter<Map<String, String>> mapJsonAdapter = moshi.adapter(
      Types.newParameterizedType(Map.class, String.class, String.class));

  public void run() throws Exception {
    Map<String, String> requestBody = new LinkedHashMap<>();

  /** Names leading to the current location in the ASN.1 document. */
  private val path = mutableListOf<String>()

  /**
   * False unless we made a recursive call to [write] at the current stack frame. The explicit box
   * adapter can clear this to synthesize non-constructed values that are embedded in octet strings.
   */
  var constructed = false

  fun write(
    name: String,
    tagClass: Int,
    tag: Long,

import java.io.Serializable;
import java.nio.ByteBuffer;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

/**
 * {@link HashFunction} adapter for {@link MessageDigest} instances.
 *
 * @author Kevin Bourrillion
 * @author Dimitris Andreou
 */
@Immutable
final class MessageDigestHashFunction extends AbstractHashFunction implements Serializable {

import java.nio.ByteBuffer;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import javax.crypto.Mac;

/**
 * {@link HashFunction} adapter for {@link Mac} instances.
 *
 * @author Kurt Alfred Kluever
 */
@Immutable
final class MacHashFunction extends AbstractHashFunction {

  @SuppressWarnings("Immutable") // cloned before each use

/// tip

All this might seem contrived. And it might not be very clear how is it useful yet.

These examples are intentionally simple, but show how it all works.

In the chapters about security, there are utility functions that are implemented in this same way.

If you understood all this, you already know how those utility tools for security work underneath.

///

type headerRewriter struct{}

// Clean up IP in case if it is ipv6 address and it has {zone} information in it, like
// "[fe80::d806:a55d:eb1b:49cc%vEthernet (vmxnet3 Ethernet Adapter - Virtual Switch)]:64692"
func ipv6fix(clientIP string) string {
	return strings.Split(clientIP, "%")[0]
}

func (rw *headerRewriter) Rewrite(req *http.Request) {

import java.io.Serializable;
import java.nio.ByteBuffer;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

/**
 * {@link HashFunction} adapter for {@link MessageDigest} instances.
 *
 * @author Kevin Bourrillion
 * @author Dimitris Andreou
 */
@Immutable
final class MessageDigestHashFunction extends AbstractHashFunction implements Serializable {