Java's synchronized Keyword Explained: Thread Safety, Methods, and Blocks

Java's synchronized keyword is a core tool for managing shared resource access in multithreaded programming, ensuring only one thread executes a critical section at a time. Without synchronization, concurrent operations on shared data — such as two threads withdrawing from the same bank account simultaneously — can produce inconsistent or incorrect results. The keyword can be applied to methods or code blocks, but not to classes, variables, or constructors. Synchronized methods work by having the JVM assign an object-level monitor lock, making other threads wait until the lock is released. For performance, developers can synchronize only the critical section within a method rather than the entire method body.