Sunday, July 1, 2007

A Java Builder Pattern

There's a Builder pattern that Joshua Bloch has briefly described in a couple of his "Effective Java Reloaded" sessions at Java One. This Builder is not necessarily a replacement for the original design pattern. The problems this Builder pattern can solve are too many constructors, too many constructor parameters, and over use of setters to create an object.

Here are some examples of the pattern in use. These examples create various Widgets with two required properties and several optional ones -

Widget x = new Widget.Builder("1", 1.0).
Widget y = new Widget.Builder("2", 2.0).
Widget z = new Widget.Builder("3", 4.0).

The basic idea behind the pattern is to limit the number of constructor parameters and avoid the use of setter methods. Constructors with too many parameters, especially optional ones, are ugly and hard to use. Multiple constructors for different modes are confusing. Setter methods add clutter and force an object to be mutable. Here is an class skeleton of the pattern -

public class Widget {
public static class Builder {
public Builder(String name, double price) { ... }
public Widget build() { ... }
public Builder manufacturer(String value) { ... }
public Builder serialNumber(String value) { ... }
public Builder model(String value) { ... }

private Widget(Builder builder) { ... }

Notice that Widget has no public constructor and no setters and that the only way to create a Widget is using the static inner class Widget.Builder. Widget.Builder has a constructor that takes the required properties of Widget. Widget's optional properties can be set using optional property methods on the Widget.Builder. The property methods of Widget.Builder return a reference to the builder so method calls can be chained.

A really nice feature of this pattern is the ability to do pre-creation validation of an object state. When setters are used to set object state during creation it is virtually impossible to guarantee that object has been properly created.

Here is the full source for Widget and its Builder -

public class Widget {
public static class Builder {
private String name;
private String model;
private String serialNumber;
private double price;
private String manufacturer;

public Builder(String name, double price) { = name;
this.price = price;

public Widget build() {
// any pre-creation validation here
Widget result = new Widget(name, price);
result.model = model;
result.serialNumber = serialNumber;
result.manufacturer = manufacturer;
return result;

public Builder manufacturer(String value) {
this.manufacturer = value;
return this;

public Builder serialNumber(String value) {
this.serialNumber = value;
return this;

public Builder model(String value) {
this.model = value;
return this;

private String name;
private String model;
private String serialNumber;
private double price;
private String manufacturer;

* Creates an immutable widget instance.
private Widget(String name, double price) { = name;
this.price = price;

public String toString() {
return super.toString() + " {"
+ "name="
+ getName()
+ " model="
+ getModel()
+ " serialNumber="
+ getSerialNumber()
+ " price="
+ getPrice()
+ " manufacturer="
+ getManufacturer()
+ "}";

public String getManufacturer() {
return manufacturer;

public String getModel() {
return model;

public String getName() {
return name;

public double getPrice() {
return price;

public String getSerialNumber() {
return serialNumber;

Notice that Widget's private constructor takes the required properties and that the Builder sets the optional properties. Another thing to note is that widget is an immutable object as implemented.


JLChoike said...

Which JDK version did you use for this example? I can compile this example in JDK 1.5, but 1.4.2 complains about not finding the Builder constructor.

Anonymous said...

This is not a builder pattern.

this is a builder pattern

Anonymous said...

Wow. That's a pretty nice design pattern. It could be extremely useful. However, it doesn't appear to be thread-safe. I think the fields of a static class like this could get modified by other threads want to do the same thing. It could be quite a surprise when executing build().

Maybe someone has a suggestion for making it safer, but I can't think of how.

Amit said...

It is threadsafe actually. The fact that you are invoking
"Widget x = new Widget.Builder("1", 1.0).model("1").build();"

implies that you are creating a new instance of the Widget.Builder every time

Amit said...

It is threadsafe actually. The fact that you are invoking
"Widget x = new Widget.Builder("1", 1.0).model("1").build();"

implies that you are creating a new instance of the Widget.Builder every time

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Rasori said...

I decided to implement this for a JTextField Document builder throughout my application. Thanks for the tip! I've linked and referenced you at my programming blog, GlitchHound.

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Tiff N Joe said...

I agree with the Anonymous poster. I'm not sure how you see this as being thread safe until you call build method.

pavolb said...

"static" does not have anything to do with not being threadsafe here. It is just keyword to define "static" inner class - that is class does not have reference to outer class. It is poor choice of keyword from this point of view, because it confuses people.

It "is" thread safe, because it is not expected you will pass instance of Builder to different thread - just create Widget and forget about it.

Notice all attributes of Builder are private and not static. Thus nobody can access them until build() is called and a Widget is constructed.

Let's split following example to its parts:

new Widget.Builder("1", 1.0). model("1").build();

1. new Widget.Builder("1", 1.0) creates an instance of Builder with initial data for name and price

2. instance.model("1") sets a new value into Builder instance

3. creates widget.

4. the instance of Builder is lost

Software Development Mumbai said...

Thank you for the great info and also a nice design pattern.

Tim Azzopardi said...

This is great. Its repetitive in that the builder repeats the fields so using a base class for both Builder and built class helps. Here's an example that is less repetitive:
Sorry in advance about the formatting!

public class CellBase {

protected CellBase() {

protected String column = "UNKNOWN";
protected Integer row = -1;
protected String value;
protected boolean editable = false;
protected String htmlTd = "";
protected boolean nowrap = true;

protected void copy(CellBase copyFrom) {
this.column = copyFrom.column;
this.row = copyFrom.row;
this.value = copyFrom.value;
this.editable = copyFrom.editable;
this.htmlTd = copyFrom.htmlTd;
this.nowrap = copyFrom.nowrap;

public String getColumn() {
return column;

public Integer getRow() {
return row;

public String getValue() {
return value;

public boolean isEditable() {
return editable;

public String getHtmlTd() {
return htmlTd;

public boolean isNowrap() {
return nowrap;


public class Cell extends CellBase {

private Cell() {

* Required for bean spring binding on the jsp
* @param value
public void setValue(String value) {
this.value = value;

public static class Builder extends CellBase {

public Builder() {

public Builder(Cell initializeFromCell) {

public Cell build() {
// any pre-creation validation here
Cell result = new Cell();
return result;

public Cell.Builder column(String column) {
this.column = column;
return this;

public Cell.Builder row(Integer row) {
this.row = row;
return this;

public Cell.Builder value(String value) {
this.value = value;
return this;

public Cell.Builder editable(boolean editable) {
this.editable = editable;
return this;

public Cell.Builder htmlTd(String htmlTd) {
this.htmlTd = htmlTd;
return this;

public Cell.Builder nowrap(boolean nowrap) {
this.nowrap = nowrap;
return this;



// Example usage
// new Cell.Builder().value("123.4567").htmlTd("align='right'").build()

Anonymous said...

This pattern looks extremely useful. I do, however, see one potential problem: it couples the Widget's clients to the Widget's build dependencies. This is an issue if the build algorithm has complex dependencies--perhaps on a properties file, a heavyweight database, etc.

The Gang of Four Abstract Factory pattern solves that problem by having the client use the factory through an interface. However, that pattern does not address constructor parameter scaling like the Bloch builder does.

Is there a way to get the best of both worlds, i.e., a modification of the Bloch pattern that would decouple Widget clients from Widget build dependencies?



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David Moles said...

(Wow, what a lot of spam comments!)

I posted about this on Stack Overflow, but since this seems to be the go-to search result on Bloch's Builder pattern, I thought I should post here, too.

There are a couple of places where your version varies from Bloch's, and I think Bloch's version is superior.

First, you have Widget's fields set in rather than the Widget constructor, which means they can't be final. This makes it harder to enforce Widget's immutability.

Second, you validate the Builder fields before constructing the Widget, instead of validating the Widget fields after constructing it. This seems intuitive, and I used to do it myself. However, Bloch points out (EJ 2ed Item 39) that if you do this, another thread can come along and muck with the Builder fields after the validation and before the constructor call (even if you're smart and don't share the Builder among threads, if any of the values in it are mutable , they're at risk) so it's safer to validate afterwards.

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cheap football jerseys said...

"Static" no not thread safe here. This is key to define "static" inner class - this class is no reference outside class. This is a poor choice of key words from this point, because it confusing people.
It is "thread safe, because it is not expected to you by different threads example construction unit -- just create small parts, and then forget it.

Kango_V said...

To adhere to the more stricter builder pattern, we have the constructor take the builder as an argument. This way you can have the required fields as final. The fields are then check in the constructor so avoiding the threading issues previously talked about.

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