Java Can Computeifabsent Generate Multiple Key Entries

Posted on 06.12.202022.08.2017by admin

A ConcurrentHashMap can be used as scalable frequency map (a form of histogram or multiset) by using LongAdder values and initializing via computeIfAbsent. For example, to add a count to a ConcurrentHashMap freqs, you can use freqs.computeIfAbsent(k - new LongAdder).increment. Aug 27, 2011 Please Rate, Comment and Subscribe.

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Introduction

Java Keytool is a key and certificate management tool that is used to manipulate Java Keystores, and is included with Java. A Java Keystore is a container for authorization certificates or public key certificates, and is often used by Java-based applications for encryption, authentication, and serving over HTTPS. Its entries are protected by a keystore password. A keystore entry is identified by an alias, and it consists of keys and certificates that form a trust chain.

This cheat sheet-style guide provides a quick reference to keytool commands that are commonly useful when working with Java Keystores. This includes creating and modifying Java Keystores so they can be used with your Java applications.

How to Use This Guide:

If you are not familiar with certificate signing requests (CSRs), read the CSR section of our OpenSSL cheat sheet
This guide is in a simple, cheat sheet format–self-contained command line snippets
Jump to any section that is relevant to the task you are trying to complete (Hint: use the Contents menu on the bottom-left or your browser’s Find function)
Most of the commands are one-liners that have been expanded to multiple lines (using the symbol) for clarity

Creating and Importing Keystore Entries

This section covers Java Keytool commands that are related to generating key pairs and certificates, and importing certificates.

Generate Keys in New/Existing Keystore

Use this method if you want to use HTTP (HTTP over TLS) to secure your Java application. This will create a new key pair in a new or existing Java Keystore, which can be used to create a CSR, and obtain an SSL certificate from a Certificate Authority.

This command generates a 2048-bit RSA key pair, under the specified alias (domain), in the specified keystore file (keystore.jks):

If the specified keystore does not already exist, it will be created after the requested information is supplied. This will prompt for the keystore password (new or existing), followed by a Distinguished Name prompt (for the private key), then the desired private key password.

Generate CSR For Existing Private Key

Use this method if you want to generate an CSR that you can send to a CA to request the issuance of a CA-signed SSL certificate. It requires that the keystore and alias already exist; you can use the previous command to ensure this.

This command creates a CSR (domain.csr) signed by the private key identified by the alias (domain) in the (keystore.jks) keystore:

After entering the keystore’s password, the CSR will be generated.

Import Signed/Root/Intermediate Certificate

Use this method if you want to import a signed certificate, e.g. a certificate signed by a CA, into your keystore; it must match the private key that exists in the specified alias. You may also use this same command to import root or intermediate certificates that your CA may require to complete a chain of trust. Simply specify a unique alias, such as root instead of domain, and the certificate that you want to import.

This command imports the certificate (domain.crt) into the keystore (keystore.jks), under the specified alias (domain). If you are importing a signed certificate, it must correspond to the private key in the specified alias:

You will be prompted for the keystore password, then for a confirmation of the import action.

Note: You may also use the command to import a CA’s certificates into your Java truststore, which is typically located in $JAVA_HOME/jre/lib/security/cacerts assuming $JAVA_HOME is where your JRE or JDK is installed.

Generate Self-Signed Certificate in New/Existing Keystore

Use this command if you want to generate a self-signed certificate for your Java applications. This is actually the same command that is used to create a new key pair, but with the validity lifetime specified in days.

This command generates a 2048-bit RSA key pair, valid for 365 days, under the specified alias (domain), in the specified keystore file (keystore.jks):

Viewing Keystore Entries

This section covers listing the contents of a Java Keystore, such as viewing certificate information or exporting certificates.

List Keystore Certificate Fingerprints

This command lists the SHA fingerprints of all of the certificates in the keystore (keystore.jks), under their respective aliases:

You will be prompted for the keystore’s password. You may also restrict the output to a specific alias by using the -alias domain option, where “domain” is the alias name.

List Verbose Keystore Contents

This command lists verbose information about the entries a keystore (keystore.jks) contains, including certificate chain length, fingerprint of certificates in the chain, distinguished names, serial number, and creation/expiration date, under their respective aliases:

You will be prompted for the keystore’s password. You may also restrict the output to a specific alias by using the -alias domain option, where “domain” is the alias name.

Note: You may also use this command to view which certificates are in your Java truststore, which is typically located in $JAVA_HOME/jre/lib/security/cacerts assuming $JAVA_HOME/ms-office-2010-home-and-business-key-generator.html. is where your JRE or JDK is installed.

Use Keytool to View Certificate Information

This command prints verbose information about a certificate file (certificate.crt), including its fingerprints, distinguished name of owner and issuer, and the time period of its validity:

You will be prompted for the keystore password.

Export Certificate

This command exports a binary DER-encoded certificate (domain.der), that is associated with the alias (domain), in the keystore (keystore.jks):

You will be prompted for the keystore password. If you want to convert the DER-encoded certificate to PEM-encoding, follow our OpenSSL cheat sheet.

Modifying Keystore

This section covers the modification of Java Keystore entries, such as deleting or renaming aliases.

Change Keystore Password

This command is used to change the password of a keystore (keystore.jks):

You will be prompted for the current password, then the new password. You may also specify the new password in the command by using the -new newpass option, where “newpass” is the password.

Delete Alias

This command is used to delete an alias (domain) in a keystore (keystore.jks):

You will be prompted for the keystore password.

Rename Alias

This command will rename the alias (domain) to the destination alias (newdomain) in the keystore (keystore.jks):

You will be prompted for the keystore password.

Conclusion

That should cover how most people use Java Keytool to manipulate their Java Keystores. It has many other uses that were not covered here, so feel free to ask or suggest other uses in the comments.

This tutorial is based on the version of keystore that ships with Java 1.7.0 update 65. For help installing Java on Ubuntu, follow this guide.

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Class

Java™ Platform
Standard Ed. 8

java.util.concurrent

Interface ConcurrentMap<K,V>

Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Superinterfaces:
Map<K,V>
All Known Subinterfaces:
ConcurrentNavigableMap<K,V>
All Known Implementing Classes:
ConcurrentHashMap, ConcurrentSkipListMap

A Map providing thread safety and atomicity guarantees.
Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread.
This interface is a member of the Java Collections Framework.
Since:
1.5

Nested Class Summary
- Nested classes/interfaces inherited from interface java.util.Map
  Map.Entry<K,V>

Method Summary

All MethodsInstance MethodsAbstract MethodsDefault Methods
Modifier and Type	Method and Description
`default V`	`compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping).
`default V`	`computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null`.
`default V`	`computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)` If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
`default void`	`forEach(BiConsumer<? super K,? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
`default V`	`getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
`default V`	`merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction)` If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
`V`	`putIfAbsent(K key, V value)` If the specified key is not already associated with a value, associate it with the given value.
`boolean`	`remove(Object key, Object value)` Removes the entry for a key only if currently mapped to a given value.
`V`	`replace(K key, V value)` Replaces the entry for a key only if currently mapped to some value.
`boolean`	`replace(K key, V oldValue, V newValue)` Replaces the entry for a key only if currently mapped to a given value.
`default void`	`replaceAll(BiFunction<? super K,? super V,? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.

Methods inherited from interface java.util.Map
clear, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, values

- Method Detail
  - getOrDefault
    Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
    Specified by:
    getOrDefault in interface Map<K,V>
    Implementation Note:
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    key - the key whose associated value is to be returned
    defaultValue - the default mapping of the key
    Returns:
    the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key
    Throws:
    ClassCastException - if the key is of an inappropriate type for this map (optional)
    NullPointerException - if the specified key is null and this map does not permit null keys (optional)
    Since:
    1.8
  - forEach
    Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.
    Specified by:
    forEach in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to, for this map:
    Implementation Note:
    The default implementation assumes that IllegalStateException thrown by getKey() or getValue() indicates that the entry has been removed and cannot be processed. Operation continues for subsequent entries.
    Parameters:
    action - The action to be performed for each entry
    Throws:
    NullPointerException - if the specified action is null
    Since:
    1.8
  - putIfAbsent
    If the specified key is not already associated with a value, associate it with the given value. This is equivalent to except that the action is performed atomically.
    Specified by:
    putIfAbsent in interface Map<K,V>
    Implementation Note:
    This implementation intentionally re-abstracts the inappropriate default provided in Map.
    Parameters:
    key - key with which the specified value is to be associated
    value - value to be associated with the specified key
    Returns:
    the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map
    NullPointerException - if the specified key or value is null, and this map does not permit null keys or values
    IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map
  - remove
    Removes the entry for a key only if currently mapped to a given value. This is equivalent to except that the action is performed atomically.
    Specified by:
    remove in interface Map<K,V>
    Implementation Note:
    This implementation intentionally re-abstracts the inappropriate default provided in Map.
    Parameters:
    key - key with which the specified value is associated
    value - value expected to be associated with the specified key
    Returns:
    true if the value was removed
    Throws:
    UnsupportedOperationException - if the remove operation is not supported by this map
    ClassCastException - if the key or value is of an inappropriate type for this map (optional)
    NullPointerException - if the specified key or value is null, and this map does not permit null keys or values (optional)
  - replace
    Replaces the entry for a key only if currently mapped to a given value. This is equivalent to except that the action is performed atomically.
    Specified by:
    replace in interface Map<K,V>
    Implementation Note:
    This implementation intentionally re-abstracts the inappropriate default provided in Map.
    Parameters:
    key - key with which the specified value is associated
    oldValue - value expected to be associated with the specified key
    newValue - value to be associated with the specified key
    Returns:
    true if the value was replaced
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map
    ClassCastException - if the class of a specified key or value prevents it from being stored in this map
    NullPointerException - if a specified key or value is null, and this map does not permit null keys or values
    IllegalArgumentException - if some property of a specified key or value prevents it from being stored in this map
  - replace
    Replaces the entry for a key only if currently mapped to some value. This is equivalent to except that the action is performed atomically.
    Specified by:
    replace in interface Map<K,V>
    Implementation Note:
    This implementation intentionally re-abstracts the inappropriate default provided in Map.
    Parameters:
    key - key with which the specified value is associated
    value - value to be associated with the specified key
    Returns:
    the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map
    NullPointerException - if the specified key or value is null, and this map does not permit null keys or values
    IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map
  - replaceAll
    Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. Exceptions thrown by the function are relayed to the caller.
    Specified by:
    replaceAll in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to, for this map: The default implementation may retry these steps when multiple threads attempt updates including potentially calling the function repeatedly for a given key.
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    function - the function to apply to each entry
    Throws:
    UnsupportedOperationException - if the set operation is not supported by this map's entry set iterator.
    NullPointerException - if function or a replacement value is null, and this map does not permit null keys or values (optional)
    ClassCastException - if a replacement value is of an inappropriate type for this map (optional)
    IllegalArgumentException - if some property of a replacement value prevents it from being stored in this map (optional)
    Since:
    1.8
  - computeIfAbsent
    If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null.
    If the function returns null no mapping is recorded. If the function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:
    Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key:
    Specified by:
    computeIfAbsent in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent: The default implementation may retry these steps when multiple threads attempt updates including potentially calling the mapping function multiple times.
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    key - key with which the specified value is to be associated
    mappingFunction - the function to compute a value
    Returns:
    the current (existing or computed) value associated with the specified key, or null if the computed value is null
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map (optional)
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
    NullPointerException - if the specified key is null and this map does not support null keys, or the mappingFunction is null
    Since:
    1.8
  - computeIfPresent
    If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value.
    If the function returns null, the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Specified by:
    computeIfPresent in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if now absent. : The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    key - key with which the specified value is to be associated
    remappingFunction - the function to compute a value
    Returns:
    the new value associated with the specified key, or null if none
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map (optional)
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
    NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null
    Since:
    1.8
  - compute
    Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping: (Method merge() is often simpler to use for such purposes.)
    If the function returns null, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Specified by:
    compute in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent: The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    key - key with which the specified value is to be associated
    remappingFunction - the function to compute a value
    Returns:
    the new value associated with the specified key, or null if none
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map (optional)
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
    NullPointerException - if the specified key is null and this map does not support null keys, or the remappingFunction is null
    Since:
    1.8
  - merge
    If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping:
    If the function returns null the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
    Specified by:
    merge in interface Map<K,V>
    Implementation Requirements:
    The default implementation is equivalent to performing the following steps for this map, then returning the current value or null if absent:
    The default implementation may retry these steps when multiple threads attempt updates including potentially calling the remapping function multiple times.
    This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
    Parameters:
    key - key with which the resulting value is to be associated
    value - the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
    remappingFunction - the function to recompute a value if present
    Returns:
    the new value associated with the specified key, or null if no value is associated with the key
    Throws:
    UnsupportedOperationException - if the put operation is not supported by this map (optional)
    ClassCastException - if the class of the specified key or value prevents it from being stored in this map (optional)
    NullPointerException - if the specified key is null and this map does not support null keys or the value or remappingFunction is null
    Since:
    1.8

Class