JVM Internals Douglas Q. Hawkins

JVM Internals Bytecode Garbage Collection Optimizations Compile Time Run Time

Java Bytecode

Stack Based Local Variable Space Local Variables Operand Stack

Operation Types Load and Store Arithmetic and Logic Type Conversion Control Transfer Object Creation and Manipulation Operand Stack Method Invocation

Demo

Garbage Collection

Generational Garbage Collection Segmented into Young, Old, and Permanent Generations Types of Collectors Parallel - across multiple threads Concurrent - while program runs

Demo

Garbage Collection Pattern MinorMajor Major Again - for objects with finalize Soft References Major Major Again - for objects with finalize Throw OutOfMemoryError

Optimizations

Optimizations Just In Time Compilation Purely Interpreted Ahead of Time Compilation Almost No Compile Time Optimization Most Optimizations are Runtime

Compile Time Demo

Is This Optimized? double sumU = 0, sumV = 0; for ( int i = 0; i < 100; ++i ) { Vector2D vector = new Vector2D( i, i ); Vector2D vector = new Vector2D( i, i ); synchronized ( vector ) { synchronized ( vector ) { sumU += vector.getU(); sumU += vector.getU(); sumV += vector.getV(); sumV += vector.getV(); }} How many...? Loop Iterations Heap Allocations Method Invocations Lock Operations

Common Sub-Expression Elimination int x = a + b; int y = a + b; int tmp = a + b; int x = tmp; int y = tmp;

Array Bounds Check Elimination int[] nums =... for ( int i = 0; i < nums.length; ++i ) { System.out.println( nums[ + i + ]= + nums[ i ] ); } int[] nums =... for ( int i = 0; i < nums.length; ++i ) { if ( i = nums.length ) { if ( i = nums.length ) { throw new ArrayIndexOutOfBoundsException(); throw new ArrayIndexOutOfBoundsException(); } System.out.println( nums[ + i + ]= + nums[ i ] ); }

Loop Invariant Hoisting for ( int i = 0; i < nums.length; ++i ) {...} int length = nums.length; for ( int i = 0; i < length; ++i ) {...}

Loop Unrolling int sum = 0; for ( int i = 0; i < 10; ++i ) { sum += i; sum += i;} int sum = 0; sum += 1;... sum += 9;

Method Inlining Vector vector =... double magnitude = vector.magnitude(); Vector vector =... double magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); vector.u*vector.u + vector.v*vector.v ); Vector vector =... double magnitude; if ( vector instance of Vector2D ) { magnitude = Math.sqrt( magnitude = Math.sqrt( vector.u*vector.u + vector.v*vector.v ); } else { magnitude = vector.magnitude(); magnitude = vector.magnitude();} static final private virtual reflective dynamic always always always often sometimes often

Lock Coarsening StringBuffer buffer =... buffer.append( Hello ); buffer.append( name ); buffer.append( \n ); StringBuffer buffer =... lock( buffer ); buffer.append( Hello ); unlock( buffer ); lock( buffer ); buffer.append( name ); unlock( buffer ); lock( buffer ); buffer.append( \n ); unlock( buffer ); StringBuffer buffer =... lock( buffer ); buffer.append( Hello ); buffer.append( name ); buffer.append( \n ); unlock( buffer );

Other Lock Optimizations Biased Locking Adaptive Locking - Thread sleep vs. Spin lock

Escape Analysis Point p1 = new Point(x1, y1), p2 = new Point(x2, y2); synchronized ( p2 ) { synchronized ( p2 ) { double dx = p1.getX() - p2.getX(); double dx = p1.getX() - p2.getX(); synchronized ( p1 ) { double dy = p1.getY() - p2.getY(); double dy = p1.getY() - p2.getY(); } } double distance = Math.sqrt( dx*dx + dy*dy ); double distance = Math.sqrt( dx*dx + dy*dy ); double dx = x1 - x2; double dx = y1 - y2; double distance = Math.sqrt( dx*dx + dy*dy );

Run Time Demo

Resources Brian Goetz Developer Works Articles Tony Printezis Garbage Collection in the Java HotSpot Virtual Machine - Garbage Collection in the Java HotSpot Virtual Machine Java Specialist Newsletter