What is Thread Safe in Java?

    What is Thread Safe in Java?

    What is Thread Safe in Java?

    Java refers to code as being thread-safe or thread-safe when it can be used or shared in a concurrent or multithreading context and will behave as intended.

    Whether you’re just starting with Java or using it for a while, you may have wondered what makes Java thread-safe. The answer is not just a straightforward answer but a combination of different factors.

    Stack class

    Stack is a class in Java that stores data in a last-in, first-out manner. There are five methods for performing operations with the stack: list iterator, addElement, remove an element, peek, and pop. It also supports an IsEmpty property.

    The ListIterator method returns a list iterator over elements in the sequence. It also throws an IndexOutOfBoundsException when an index out of range occurs. If you are doing a comparison and swapping operations, consider using optimistic locking.

    The Java Stack class also supports the Peek method, which looks for the stack’s top element and reports the object’s position. However, the peek method is not used for removing the top element from the stack.

    The addElement method works the same as the addElement method in the Vector class. It passes the object to be pushed into the stack. Next, the Object obj parameter is passed to the search method, which returns the position of the element obj from the top of the stack. If the element is not found, the Object obj is returned as a -1.

    The Peek method works the same as the Peek method in the Vector class. It returns the top element of the stack, but the Object obj parameter is not passed to the search method. If the element is not found, the Peek method throws an EmptyStackException.


    The Java Stack class also supports an IsEmpty method. It returns true if the stack is empty and false otherwise. It also provides a walk method that opens a sequential stream of StackFrames for the current thread. The walk method closes the StackFrames when it returns.

    Vector class

    Using a Vector class in Java is like using an ArrayList. The Vector class implements the List interface and uses the Enumeration interface to traverse the elements in the vector.

    The Vector class combines the resizable array and synchronization features. The capacity of the vector increases as the number of elements increases. This is a synchronized way of minimizing memory usage. It will increase the capacity in chunks of the size specified in the capacity increment argument. This way, the Vector class ensures that the number of elements is at least as large as the vector size.

    Vectors have four types of constructors. The default constructor creates a Vector with a capacity of 10. The last overloaded constructor takes a predefined collection as an argument and creates a Vector with the predefined collection.

    The Vector class has a boolean addAll method that adds a collection of elements to the vector. It also has a boolean removeAll method that deletes all elements from the vector. The Vector class also has an elemental remove method that removes an element from the vector. Finally, it also has a remote range method that removes a range of elements from the vector.

    The Vector class also has an Object hashCode method that returns the hash code of the vector. It also has a String toString method that returns the string representation of each element. The Vector class also has a boolean equals method that compares an object to a vector. It also has a boolean that contains a method that tests if a specific object is a vector component. It also has an int lastIndexOf method that returns the index of the last occurrence of the element in the vector. Finally, it also has a void ensure capacity method that increases the vector’s capacity.

    Hashtable class

    Using Hashtable, a collection, you can perform reads and writes without requiring a lock. However, you should be aware of a few drawbacks.

    Hashtable is an implementation of the Map interface, which is part of the Java Collections Framework. This interface supports multiple readers and writers concurrently. However, the enumeration operation used to iterate over the keys in Hashtable is not thread-safe. It is also a slow operation.

    In addition, Hashtable does not allow null keys. Instead, it uses a hash function to compute an index for each key, which is then mapped to a value. Any object that is not null can be used as a key. Objects that are mapped to values must have an equals method and a hashCode method. This ensures memory consistency on key/value operations in a multithreading environment.

    When creating a Hashtable object, you can use an initial capacity, which is the number of buckets you will use. It is important to note that the initial capacity is not used for growing Hashtables. You can adjust the expected concurrency for updates. This is useful for determining when the Hashtable should be resized. Changing the Hashtable capacity is a relatively slow operation.


    You can obtain the number of key-value pairs in the Hashtable with the int size method. In addition, you can use the boolean isEmpty method to check whether there are no entries in the Hashtable. If there are no entries, the boolean returns true.

    Wrapper object approach

    Embedding an object in a wrapper is an example of thread-safe Java design. It makes it possible to use an ordinary collection object in a multithreaded environment.

    The Java virtual machine (VM) allocates memory for the object. The Java program then instantiates the object. It then uses the object to access an underlying COM object.

    This object contains a pointer to the IUnknown interface, a flag for the tentative home thread, and a thread identifier. It is the home thread identifier that determines if the object is thread-safe.

    The Java program then uses the object to access a COM object. The thread system then intercepts the access and ensures that the home thread handles the request.

    In addition to the synchronized call, Java uses a wrapper object to perform other thread-safe functions. For instance, the wrapper object maps the Java reference to the underlying COM object. It also contains a cache of interface pointers.

    The wrapper object may also be given full access to the object. This can be an expensive operation, but it is necessary to make the object thread-safe.

    The wrapper object approach is one of the most common methods for making an object thread-safe in Java. However, it does not offer performance improvements over other thread-safe Java methods. For example, you may need help using shared variables with the wrapper object approach.

    ++(the increment operator) is not an atomic operation

    ++ (the increment operator) is not an atomic operation in Java. Java discourages its use of it. There are better alternatives. For example, Java uses an ATOMIC_FLAG_INIT macro to initialize objects in other contexts.

    The atomic flag type is a minimal hardware-implemented type. It can emulate the remaining types. It has two primary operations: compare and swap. It also has less-than-ideal properties. As a result, it may be too expensive for some architects.

    Using the atomic flag type is not recommended, as it has fewer ideal properties. Its primary advantage is that it is a minimum hardware-implemented type. It is also possible to use it as a proxy for atomic type.

    It is possible to perform atomic operations with more significant types, such as arrays. They are also possible to perform atomic operations with locking algorithms. However, they do require a synchronization protocol. They also provide acquire and release semantics implicitly. This is optional for most operations, and they are only helpful for a few.

    It is also possible to perform atomic operations with pointers. They are also possible to perform atomic stores and load on them. In addition, they are possible to use pointer conversions on their type. However, this requires the operand to be an arithmetic type other than cv bool. Moreover, the operand must not be null.

    It is also possible to perform a better wait operation, which requires interaction with the operating system scheduler. However, it has pathological performance characteristics on multi-programmed machines.

    The thread scheduler interferes with threads

    Even though Java does not implement real-time scheduling, the runtime system still uses a simple deterministic scheduling algorithm. It selects a thread to run on the processor based on its priority. The priority of a thread is an integer between 1 and 10.

    In the case of a single-thread program, one thread enters the program at a time. Once a thread enters the program, it is run until it preempts a thread with a higher priority. If the lower-priority thread preempts the higher-priority thread, the higher-priority thread continues to run until it finishes.

    In the case of a multi-thread program, the thread scheduler chooses the highest-priority thread. When a thread of higher priority is not ready to run, the scheduler will choose a lower-priority thread to run. If the higher-priority thread is preempted, the lower-priority thread is interrupted.

    Thread scheduling in Java is a preemptive scheme. The scheduler selects the thread to run by examining the arrival time and the thread’s priority. It also considers the time it takes for the thread to run. Depending on the system, the scheduler may run the lower-priority thread instead of the higher-priority thread.

    Priority inversion is a very undesirable phenomenon. If a higher-priority thread cannot get a lock while the lower-priority thread is waiting, the higher-priority thread blocks. The lower-priority thread then holds the lock until the higher-priority thread releases it.

    Is Java thread-safe for strings?

    The string must also be thread-safe since every immutable object in Java is. Two threads cannot utilize a string at the same time. Once assigned, a string cannot be changed.


    What does “thread-safe” mean?

    Multithreaded programs can benefit from the thread safety notion used in computer programming. Only shared data structures can be altered by thread-safe code to guarantee that each thread behaves as intended and satisfies its requirements without interfering with other threads.

    What does Java mean by synchronized and thread-safe?

    When a method or class instance is considered thread-safe, several threads can be used concurrently without encountering any issues. In contrast, synchronized means that only one thread may be active.

    What does Java mean by thread-safe and unsafe?

    Unsafe holds no instance state at the Java level. Instead, most of its methods use native code, which either doesn’t operate on a shared mutable state or uses the native allocator to allocate and free memory. Both are thread-safe operations (you typically don’t synchronize calls to new C++, for instance).

    Which class is safe for threads?

    Java’s thread-safe collection classes include Stack, Vector, Properties, Hashtable, etc.