Find a Tab in Hundreds of Tabs of Dozens of Safari Windows

2020 May 29

When use Safari in my Mac, I often keep hundreds of tabs open in dozens of Safari windows. For a recently working on tab, I may remember which Safari window it’s in, and switch to that window by looking at the snapshots of all windows brought up by Ctrl + Down arrow.

all windows

To locate a tab which I totally forget where is it in, it can be easily found via the menu bar -> Help -> Search. Type the keywords of that tab in the input box.

search tab in menubar

If you know which Safari window a tab is in, click the “double square” icon in the upper-right corner of the window (or View -> Show Tab Overview).

search tab in window

Then type some keywords of the tab to narrow down the matched result.

search tab in window
mac

Which Accessor Style? "Fluent" vs. "Java Bean"

2020 May 19

Basically, there are two styles of accessor naming convention in Java.

One is the traditional “Java Bean” style.

public class Item {
    ItemType getType();
}

Another is called the “fluent” style.

public class Item {
    ItemType type();
}

The fluent style saves some typing when writing code, also makes code a bit less verbose, item.type().

For example, the Lombok library supports this fluent style.

lombok.accessors.fluent = [true | false] (default: false)
If set to true, generated getters and setters will not be prefixed with the bean-standard ‘get, is or set; instead, the methods will use the same name as the field (minus prefixes).

Which style is better?

Actually, the verbose Java Bean style, is the better one. It’s because a lot of third party libraries are acknowledging the Java Bean style. For example, if Item is going to be serialized by the Jackson library as a JSON string, the fluent style wouldn’t work out of the box. Also, most of DTO mapping libraries are using the Java Bean style too. Therefore, using the standard Java Bean accessor style save effort when integrate our classes with other libraries and frameworks.

With the Lombok library and the auto-completion in modern IDEs, the Java Bean style doesn’t necessarily mean more typing.

Run Multiple Gradle Sub Project Tasks

2020 Apr 2

Use the brace expansion mechanism of Bash, multiple sub project tasks can be run in the command line like below.

$ ./gradlew sub-prj-1:{tasks,help}

After brace expansion, it is same as below, but saves some typing.

$ ./gradlew sub-prj-1:tasks sub-prj-1:help

More examples.

$ ./gradlew sub-prj-{1,2}:test
# same as
# ./gradlew sub-prj-1:test sub-prj-2:test

$ ./gradlew sub-prj-{1,2}:{clean,build}
# same as
# ./gradlew sub-prj-1:clean sub-prj-1:build sub-prj-2:clean sub-prj-2:build

The if-else Control Flow Using Optional

2020 Mar 27

Sometimes you may want to write the if-else control flow based on an Optional object.

For example, an API from a third party library declares Optional as its return type. You need to compose an if-else like control flow using that Optional object. Of course, it can be done by testing isPresent() in a traditional if-else statement.

var itemOpt = service.getItem(itemId);
if (itemOpt.isPresent()) {
    addToOrder(itemOpt.get());
} else {
    log.info("missing item {}", itemId);
    sendOutItemMissedEvent();
}

The above code doesn’t take any advantage of Optional. Actually, since Java 9 the ifPresentOrElse​(Consumer<? super T>, Runnable) method can be used to implement such control flow, which is a bit more elegant.

service.getItem(itemId).ifPresentOrElse(
    item -> {
        addToOrder(item);
    },
    () -> {
        log.info("missing item {}", itemId);
        sendOutItemMissedEvent();
    });

Print Exception Together With Parameterized Log Messages

2020 Mar 5

Often when use SLF4J, you may wonder if an exception object can be printed in the same parameterized log message together with other objects. Actually SLF4J supports it since 1.6.0.

log.error("failed to add item {}", "item-id-1", new RuntimeException("failed to add item"));

The SLF4J call above prints log message like below.

13:47:16.119 [main] ERROR example.TmpTest - failed to add item item-id-1
java.lang.RuntimeException: failed to add item
    at example.TmpTest.logErrorTest(TmpTest.java:10)
    ...
    ...

So it actually doesn’t need to log as below.

log.error("failed to add item {}", "item-id-1");
log.error("exception details:", new RuntimeException("failed to add item"));

Here is what the official FAQ says.

The SLF4J API supports parametrization in the presence of an exception, assuming the exception is the last parameter.

Use the first style to save one line.

SLF4J doesn’t clearly state this behavior in the Javadoc of Logger.error(String format, Object... arguments) (at least not in the 1.7.26 version). Maybe if a new method like below is added to Logger, programmer would find out this feature more easily.

public void error(String format, Throwable t, Object... arguments);

Process Substitution

2019 Dec 19

From man bash, for “process substitution” it says

It takes the form of <(list) or >(list)

Note: there is no space between < or > and (.

Also, it says,

The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion.

The <(list) Form

$ diff <(echo a) <(echo b)
1c1
< a
---
> b

Usually the diff takes two files and compares them. The process substitution here, <(echo a), creates a file in /dev/fd, for example /dev/fd/63. The stdout of echo a command is connected to /dev/fd/63. Meanwhile, /dev/fd/63 is used as an input file/parameter of diff command. Similar for <(echo b). After Bash does the substitution, the command is like diff /dev/fd/63 /dev/fd/64. In diff’s point of view, it just compares two normal files.

In this example, one advantage of process substitution is eliminating the need of temporary files, like

$ echo a > tmp.a && echo b > tmp.b \ 
    && diff tmp.a tmp.b            \
    && rm tmp.{a,b}

The >(list) Form

$ echo . | tee >(ls)

Similar, Bash creates a file in /dev/fd when it sees >(ls). Again, let’s say the file is /dev/fd/63. Bash connects /dev/fd/63 with stdin of ls command, also the file /dev/fd/63 is used as a parameter of tee command. The tee views /dev/fd/63 as a normal file. tee writes content, here is ., into the file, and the content will “pipe” into the stdin of ls.

Compare with Pipe

Pipe, cmd-a | cmd-b, basically just passes stdout of the command on the left to the stdin of the command on the right. Its data flow is restricted, which is from left to right.

Process substitution has more freedom.

# use process substitution
$ grep -f <(echo hello) file-a
hello
# use pipe
$ echo hello | xargs -I{} grep {} file-a
hello

And for commands like diff <(echo a) <(echo b), it’s not easy to be done by pipe.

More Examples

$ paste <(echo hello) <(echo world)
hello   world

How it works

From man bash,

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

Read more about named pipe and /dev/fd.

For /dev/fd,

The main use of the /dev/fd files is from the shell. It allows programs that use pathname arguments to handle standard input and standard output in the same manner as other pathnames.

In my OS (Ubuntu), the Bash uses /dev/fd for process substitution.

$ ls -l <(echo .)
lr-x------ 1 user user 64 12月 19 11:19 /dev/fd/63 -> pipe:[4926830]

Bash replaces <(echo .) with /dev/fd/63. The above command is like ls -l /dev/fd/63.

Or find the backing file via,

$ echo <(true)
/dev/fd/63

(After my Bash does the substitution, the command becomes echo /dev/fd/63, which outputs /dev/fd/63.)

Deploy a Rails App with Docker

2019 Aug 14

This site is deployed using Docker containers on a small DigitalOcean Droplet.

Since I never have a private Docker registry, the original deploy flow was,

  • push the source code to the remote Git repository
  • on the Droplet, fetch the source code from the Git remote
  • build an Docker image from the source code
  • start the Docker container

Since some time ago, the docker build has kept failing on the bundle install instruction on the Droplet. During bundle install, the Bundler fails to build the sassc gem. The Bundler complains that,

virtual memory exhausted: Cannot allocate memory

, while building the sassc gem.

A workaround I tried was,

  • build the Docker image on a more powerful machine, like the local dev machine
  • save the image into a tar archive (docker save img-foo | bzip2 > foo.img.bz)
  • copy the tar to the Droplet
  • load the image from the tar (bunzip2 foo.img.bz && docker load -i foo.img)
  • start the Docker container

One problem of the workaround was the volume to transfer is a bit large (the image is around 1G before compression, and 370M after compression). It was time consuming over a slow network.

Current Deploy Workflow

Actually, Bundler supports fetching gems from local cache, not only from rubygems.org. Using bundle package --all can,

Copy all of the .gem files needed to run the application into the vendor/cache directory.

In the future, when running bundle install, use the gems in the cache in preference to the ones on rubygems.org.

The sassc gem is built on the local machine while bundle package, and put into vendor/cache directory.

Now, the deploy flow is like,

  • run bundle package --all if the application’s dependencies change
  • check in vendor/cache folder and push it to the Git remote
  • on the Droplet, fetch the source code from the Git remote
  • build an Docker image from the source code
  • start the Docker container

The Dockerfile is like,

USER app
RUN mkdir -p /home/app/source-code
WORKDIR /home/app/source-code
COPY vendor/cache ./vendor/cache/
COPY Gemfile* ./
# use RVM, so "bash -lc ..."
RUN bash -lc 'bundle install --deployment --without development test'
# copy the app source code
COPY . ./

Other Notes

If a gem is located at a particular git repository using the :git parameter, like

gem "rails", "2.3.8", :git => "https://github.com/rails/rails.git"

Then the --all option should be used in bundle package.

Some gems are built using CMake. When run bundle package --all, CMakeCache.txt files are created and stored in vendor/cache. The problem with CMakeCache.txt files is that they use hardcoded paths of the machine runs bundle package --all. When build the Docker image in the remote server, the hardcoded paths fail the bundle install. Therefore, in the .dockerignore file, add the following line.

vendor/cache/*/CMakeCache.txt

Java Generics, Bounded Wildcards and PECS

2019 Aug 8

With a class hierarchy below,

class Fruit {
    public void hello() {}
}
class Apple extends Fruit {}
class Banana extends Fruit {}

Lower Bounded Wildcards and Upper Bounded Wildcards

? super Fruit is called lower bounded wildcard.

? extends Fruit is called upper bounded wildcard.

Usually a class hierarchy is illustrated like,

   Object
     |
   Fruit
   /   \
Apple Banana

The base class is placed upper, subclasses placed lower.

For ? super Fruit, the lower bound of the type argument is determined, which is Fruit, so it’s called lower bounded wildcard. Similar, for type argument ? extends Fruit, its upper bound is fixed, so it’s called upper bounded wildcard.

”? super Fruit”

void func(List<? super Fruit> fruits) {
    fruits.add(new Fruit()); // ok
    fruits.add(new Apple()); // ok
    fruits.add(new Banana()); // ok
    // Error: Cannot resolve method 'hello'
    fruits.get(0).hello();  
}

The ? in the declaration List<? super Fruit> means fruits is a List of an unknown type. The ? super Fruit means the unknown type is limited to be Fruit or its super class (Object in this case). When invoke func(List<? super Fruit> ), List<Fruit> or List<Object> can be passed as the argument.

All the fruits.add(...) in the above code is ok, because no matter what type actually the type argument is Fruit, Apple, and Banana are the subtypes of that type. An Apple object is a Fruit (or Object) object, so an Apple object can be added into a List of Fruit (or Object).

Why fruits.get(0).hello() fails?

The actual type of the elements in the fruits List is undetermined when declare the method. When call a method on an element, the compiler needs to make sure the unknown type has the method. ? super Fruit in the type argument declaration sets bounds for the unknown type, one bound (“lower”) is type Fruit, the other bound (“upper”) is type Object To safely invoke methods on the unknown type, only methods of the most upper type can be invoked. Obviously, method hello is not from the most upper type (i.e. Object). If passing a List<Object> as the parameter, then obviously the elements in fruits would not have hello() method. Therefore the compiler raises the error.

”? extends Fruit”

void func(List<? extends Fruit> fruits){
    // Error: add(capture<? extends Fruit>) in List cannot be applied to add(Fruit)
    fruits.add(new Fruit()); 
    fruits.add(new Apple()); // similar error
    fruits.add(new Banana()); // similar error
    fruits.get(0).hello(); // ok
}

Similar, declaring the parameter as List<? extends Fruit> means fruits is an unknown type, and can be List<Fruit>, List<Apple> or List<Banana> when the method is invoked.

Why fruits.get(0).hello() is ok in this case?

Similar, ? extends Fruit sets bounds for the unknown type, the upper bound is determined as the type Fruit. The hello() method is from the most upper type, so the compiler knows it’s safe to call it within the func body.

Why statements like fruits.add(new Fruit()) fails?

The type of fruits is unknown, it doesn’t mean the type of fruits is “dynamic”. Being “dynamic” means the type of fruits can be List<Fruit> at some point, then be List<Apple> at some other point. It’s not possible in Java, since Java is a static language. The two types, List<Fruit> and List<Apple>, have nothing to do with each other (List<Apple> is not a subtype of List<Fruit>). The type of the fruits parameter is determined on the invocation of func. For a specific invocation, the type of fruits is determined and static. For example, with an invocation like func(new ArrayList<Apple>()), the statement fruits.add(new Fruit()) would raise compiler error (since a Fruit cannot be add()ed into a List<Apple>). To ensure all the possible invocation of func works, the compiler just can’t allow the statements like fruits.add(new Fruit()) appear in the method body.

What is “capture” (capture<? extends Fruit>)?

"ssh" to a Windows Host

2019 Jun 25

Every now and then, we may want to connect to a Windows host via ssh. Maybe it’s due to the frustration on connecting a Windows host via Remote Desktop Connection over a slow network.

Below is a way to ssh (not really, actually) to a Windows host.

Step 0, install a Linux virtual machine in the Windows host

I use Oracle Virtual Box, and install a Ubuntu guest.

Step 1, share folders between the Windows host and the guest

Share the working folders between the Windows and the Ubuntu, so that these folders are accessible within the guest. Just google “virtualbox share folder”.

NAT

Step 2, configure the guest’s network settings

Choose NAT as the adapter type.

NAT

In the “Advanced” settings, add a port forwarding rule. Choose an available port in the Windows host for “Host Port”. Set “Guest Port” to 22, which is the default port sshd listening on in the Ubuntu guest.

port forwarding

To find an available port in the Windows host, run netsh firewall show state in the cmd.exe. In my corporate network, for example, port 443 is allowed.

Step 3, make a connection

Open a ssh client in another machine, type the IP (or host name) of the Windows host as the hostname, use the “Host Port” as the port, 443 in my case, for the ssh connection.

# my_user is the username of the Ubuntu guest VM
# 1.2.3.4 is the IP of the Windows host
# 443 is the opened port in the WIndows host
ssh my_user@1.2.3.4 -p 443
# use the hostname of the Windows machine is also ok
ssh my_user@my_win_desktop.my_corp.com -p 443

After the connection is up, we’re in the Ubuntu VM. Change the directory to one of the shared folders set up in step 1, and do the work. Much better user experience than Windows Remote Desktop when the network is slow.

Other ways

One way is to set the adapter as “Bridged Adapter” for the Ubuntu guest, so that the guest get an IP from the corporate network. Configure the port of sshd to a port the corporate network allows to connect from outside. One inconvenience is that sometimes the corporate network may assign a new IP for the guest after reboot.

Another way is to install the OpenSSH Server directly in the Windows host.

Daily Dev Log: "--help" vs. "man"

2019 Jun 24

We just can’t remember options of CLI tools. In most cases, --help, like grep --help, is the go-to way to look for help.

For example, if you forget what -H of grep does.

$ grep --help | grep -- -H
  -H, --with-filename       print the file name for each match

man is too formal, wordy and overwhelming, comparing with --help. Usually, we can find out most of what we want in the output of --help, without turning to man.

In some environments, man pages may not be available there. One such example is Git Bash for Windows. Commands in Git Bash for Windows don’t have man pages. Relying on man only means you have to google the man(ual) in the browsers.

To save some typing, a bash function can also be added into the ~/.bashrc.

h() { $1 --help; }

Then type h grep to show the help.

Note that different commands or different variants of a command may print help text in different verbosity. For example, the builtin grep in OS X prints help text like below.

$ grep --help
usage: grep [-abcDEFGHhIiJLlmnOoqRSsUVvwxZ] [-A num] [-B num] [-C[num]]
	[-e pattern] [-f file] [--binary-files=value] [--color=when]
	[--context[=num]] [--directories=action] [--label] [--line-buffered]
	[--null] [pattern] [file ...]

It’s much less than the Linux grep’s help text. (Most of CLI tools support this level of help text at least.)

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