How does parallel test execution work in JUnit 6?

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

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

# Strategy: dynamic (uses available processors)
junit.jupiter.execution.parallel.config.strategy=dynamic
junit.jupiter.execution.parallel.config.dynamic.factor=1.5

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

# Default mode: all test classes run sequentially,
# all methods within a class run sequentially.
# Override with:
junit.jupiter.execution.parallel.mode.default=concurrent
junit.jupiter.execution.parallel.mode.classes.default=concurrent

// @Execution annotation: fine-grained control per class or method
import org.junit.jupiter.api.parallel.*;

@Execution(ExecutionMode.CONCURRENT)  // this class runs in parallel
class FastParallelTest {
    @Test void first()  { ... }
    @Test void second() { ... }
}

@Execution(ExecutionMode.SAME_THREAD)  // always sequential
class OrderDependentTest {
    @Test @Order(1) void setup()  { ... }
    @Test @Order(2) void execute(){ ... }
}

// @ResourceLock: prevent concurrent access to shared resources
class DatabaseTest {
    @Test
    @ResourceLock(value = "database", mode = ResourceAccessMode.READ_WRITE)
    void writesToDatabase() { ... }

    @Test
    @ResourceLock(value = "database", mode = ResourceAccessMode.READ)
    void readsFromDatabase() { ... }
    // Reads run concurrently with each other
    // Writes are exclusive (no concurrency with reads or other writes)
}