Java设计模式中的建造者模式详解

 
public class ResourcePoolConfig {
  private static final int DEFAULT_MAX_TOTAL = 8;
  private static final int DEFAULT_MAX_IDLE = 8;
  private static final int DEFAULT_MIN_IDLE = 0;
  private String name;
  private int maxTotal = DEFAULT_MAX_TOTAL;
  private int maxIdle = DEFAULT_MAX_IDLE;
  private int minIdle = DEFAULT_MIN_IDLE;
  public ResourcePoolConfig(String name, Integer maxTotal, Integer maxIdle, Integer minIdle) {
    if (StringUtils.isBlank(name)) {
      throw new IllegalArgumentException("name should not be empty.");
    }
    this.name = name;
    if (maxTotal != null) {
      if (maxTotal <= 0) {
        throw new IllegalArgumentException("maxTotal should be positive.");
      }
      this.maxTotal = maxTotal;
    }
    if (maxIdle != null) {
      if (maxIdle < 0) {
        throw new IllegalArgumentException("maxIdle should not be negative.");
      }
      this.maxIdle = maxIdle;
    }
    if (minIdle != null) {
      if (minIdle < 0) {
        throw new IllegalArgumentException("minIdle should not be negative.");
      }
      this.minIdle = minIdle;
    }
  }
  //...省略getter方法...
}

// ResourcePoolConfig使用举例
ResourcePoolConfig config = new ResourcePoolConfig("dbconnectionpool");
config.setMaxTotal(16);
config.setMaxIdle(8);

 
public class ResourcePoolConfig {
  private String name;
  private int maxTotal;
  private int maxIdle;
  private int minIdle;
  private ResourcePoolConfig(Builder builder) {
    this.name = builder.name;
    this.maxTotal = builder.maxTotal;
    this.maxIdle = builder.maxIdle;
    this.minIdle = builder.minIdle;
  }
  //...省略getter方法...
  //我们将Builder类设计成了ResourcePoolConfig的内部类。
  //我们也可以将Builder类设计成独立的非内部类ResourcePoolConfigBuilder。
  public static class Builder {
    private static final int DEFAULT_MAX_TOTAL = 8;
    private static final int DEFAULT_MAX_IDLE = 8;
    private static final int DEFAULT_MIN_IDLE = 0;
    private String name;
    private int maxTotal = DEFAULT_MAX_TOTAL;
    private int maxIdle = DEFAULT_MAX_IDLE;
    private int minIdle = DEFAULT_MIN_IDLE;
    public ResourcePoolConfig build() {
      // 校验逻辑放到这里来做，包括必填项校验、依赖关系校验、约束条件校验等
      if (StringUtils.isBlank(name)) {
        throw new IllegalArgumentException("...");
      }
      if (maxIdle > maxTotal) {
        throw new IllegalArgumentException("...");
      }
      if (minIdle > maxTotal || minIdle > maxIdle) {
        throw new IllegalArgumentException("...");
      }
      return new ResourcePoolConfig(this);
    }
    public Builder setName(String name) {
      if (StringUtils.isBlank(name)) {
        throw new IllegalArgumentException("...");
      }
      this.name = name;
      return this;
    }
    public Builder setMaxTotal(int maxTotal) {
      if (maxTotal <= 0) {
        throw new IllegalArgumentException("...");
      }
      this.maxTotal = maxTotal;
      return this;
    }
    public Builder setMaxIdle(int maxIdle) {
      if (maxIdle < 0) {
        throw new IllegalArgumentException("...");
      }
      this.maxIdle = maxIdle;
      return this;
    }
    public Builder setMinIdle(int minIdle) {
      if (minIdle < 0) {
        throw new IllegalArgumentException("...");
      }
      this.minIdle = minIdle;
      return this;
    }
  }
}
// 这段代码会抛出IllegalArgumentException，因为minIdle>maxIdle
ResourcePoolConfig config = new ResourcePoolConfig.Builder()
        .setName("dbconnectionpool")
        .setMaxTotal(16)
        .setMaxIdle(10)
        .setMinIdle(12)
        .build();