Builder Pattern

建造者模式

这种模式太常见了，开源框架中的各种builder都是

mybatis中

org.apache.ibatis.mapping.Environment

public final class Environment {
  private final String id;
  private final TransactionFactory transactionFactory;
  private final DataSource dataSource;

  public Environment(String id, TransactionFactory transactionFactory, DataSource dataSource) {
    if (id == null) {
      throw new IllegalArgumentException("Parameter 'id' must not be null");
    }
    if (transactionFactory == null) {
      throw new IllegalArgumentException("Parameter 'transactionFactory' must not be null");
    }
    this.id = id;
    if (dataSource == null) {
      throw new IllegalArgumentException("Parameter 'dataSource' must not be null");
    }
    this.transactionFactory = transactionFactory;
    this.dataSource = dataSource;
  }

  public static class Builder {
    private final String id;
    private TransactionFactory transactionFactory;
    private DataSource dataSource;

    public Builder(String id) {
      this.id = id;
    }

    public Builder transactionFactory(TransactionFactory transactionFactory) {
      this.transactionFactory = transactionFactory;
      return this;
    }

    public Builder dataSource(DataSource dataSource) {
      this.dataSource = dataSource;
      return this;
    }

    public String id() {
      return this.id;
    }

//  最终返回Environment对象
    public Environment build() {
      return new Environment(this.id, this.transactionFactory, this.dataSource);
    }

  }

  public String getId() {
    return this.id;
  }

  public TransactionFactory getTransactionFactory() {
    return this.transactionFactory;
  }

  public DataSource getDataSource() {
    return this.dataSource;
  }

}