Much as memory is managed in Java, an engineer may need to perform as much optimization as possible to minimize latency and maximize throughput, in critical applications. Much as it is IMPOSSIBLE to explicitly control when garbage collection is triggered in the JVM, it is possible to influence how it occurs as regards the OBJECTS we have created.

Java provides us with reference objects to control the relationship between the objects we create and the garbage collector.

By default, every object we create in a Java program is strongly referenced by a VARIABLE:

StringBuilder sb = new StringBuilder();

In the above snippet, the new keyword creates a new StringBuilder object and stores it on the heap. The variable sb then stores a strong reference to this object. What this means for the garbage collector is that the particular StringBuilder object is not eligible for collection at all due to a strong reference held to it by sb. The story only changes when we nullify sb like this:

sb = null;

After calling the above line, the object will then be eligible for collection.

We can change this relationship between the object and the garbage collector by explicitly wrapping it INSIDE another reference object which is located inside java.lang.ref package.

A soft reference can be created to the above object like this:

StringBuilder sb = new StringBuilder();

SoftReference<StringBuilder> sbRef = new SoftReference<>(sb);

sb = null;

In the above snippet, we have created two references to the StringBuilder object. The first line creates a strong reference sb and the second line creates a soft reference sbRef. The third line should make the object eligible for collection but the garbage collector will postpone collecting it because of sbRef.

The story will only change when memory becomes tight and the JVM is on the brink of throwing an OutOfMemory error. In other words, objects with only soft references are collected as a last resort to recover memory.

A weak reference can be created in a similar manner using Weak Reference class. When sb is set to null and the StringBuilder object only has a weak reference, the JVM’s garbage collector will have absolutely no compromise and immediately collect the object at the very next cycle.

A phantom reference is similar to a weak reference and an object with only phantom references will be collected without waiting. HOWEVER, phantom references are enqueued as soon as their objects are collected. We can poll the reference queue to know exactly when the object was collected.

Much as memory is managed in Java, an engineer may need to perform as much optimization as possible to minimize latency and maximize throughput, in critical applications. Much as it is impossible to explicitly control when garbage collection is triggered in the JVM, it is possible to influence how it occurs as regards the objects we have created.

Java provides us with reference objects to control the relationship between the objects we create and the garbage collector.

By default, every object we create in a Java program is strongly referenced by a variable:

StringBuilder sb = new StringBuilder();

In the above snippet, the new keyword creates a new StringBuilder object and stores it on the heap. The variable sb then stores a strong reference to this object. What this means for the garbage collector is that the particular StringBuilder object is not eligible for collection at all due to a strong reference held to it by sb. The story only changes when we nullify sb like this:

sb = null;

After calling the above line, the object will then be eligible for collection.

We can change this relationship between the object and the garbage collector by explicitly wrapping it inside another reference object which is located inside java.lang.ref package.

A soft reference can be created to the above object like this:

StringBuilder sb = new StringBuilder();

SoftReference<StringBuilder> sbRef = new SoftReference<>(sb);

sb = null;

In the above snippet, we have created two references to the StringBuilder object. The first line creates a strong reference sb and the second line creates a soft reference sbRef. The third line should make the object eligible for collection but the garbage collector will postpone collecting it because of sbRef.

The story will only change when memory becomes tight and the JVM is on the brink of throwing an OutOfMemory error. In other words, objects with only soft references are collected as a last resort to recover memory.

A weak reference can be created in a similar manner using Weak Reference class. When sb is set to null and the StringBuilder object only has a weak reference, the JVM’s garbage collector will have absolutely no compromise and immediately collect the object at the very next cycle.

A phantom reference is similar to a weak reference and an object with only phantom references will be collected without waiting. However, phantom references are enqueued as soon as their objects are collected. We can poll the reference queue to know exactly when the object was collected.