How does parallel test execution work in JUnit 6?

# junit-platform.properties (in src/test/resources/)

# Enable parallel execution
junit.jupiter.execution.parallel.enabled=true

# Strategy: DYNAMIC (based on CPU count) or FIXED (explicit thread count)
junit.jupiter.execution.parallel.config.strategy=dynamic
junit.jupiter.execution.parallel.config.dynamic.factor=2
# thread count = processor count * factor  (e.g. 4 cores * 2 = 8 threads)

# OR: fixed thread count
# junit.jupiter.execution.parallel.config.strategy=fixed
# junit.jupiter.execution.parallel.config.fixed.parallelism=4

# Default execution mode: SAME_THREAD (sequential) or CONCURRENT
junit.jupiter.execution.parallel.mode.default=concurrent
junit.jupiter.execution.parallel.mode.classes.default=concurrent

// Fine-grained control with @Execution:
import org.junit.jupiter.api.parallel.*;

@Execution(ExecutionMode.CONCURRENT)  // This class runs in parallel
class ParallelTests {

    @Test void test1() { /* can run in parallel with test2 */ }
    @Test void test2() { /* can run in parallel with test1 */ }
}

@Execution(ExecutionMode.SAME_THREAD)  // This class runs sequentially
class SequentialTests {
    // Useful for tests with shared mutable state or DB transactions
}

// @ResourceLock: prevent concurrent access to named resources
@ResourceLock(value = "database", mode = ResourceAccessMode.READ_WRITE)
void dbWriteTest() { /* exclusive DB access */ }

@ResourceLock(value = "database", mode = ResourceAccessMode.READ)
void dbReadTest() { /* shared read access -- can run with other READ tests */ }